Walter Payton, the Chicago Bears' running back whose single-minded aggressiveness and singular combination of power and speed made him the leading ground gainer in pro football history, died yesterday at his home in Barrington, Ill., a Chicago suburb. He was 45.

The cause was bile duct cancer, said Dr. Greg Gores, Payton's physician at the Mayo Clinic in Rochester, Minn.

On Feb. 2, during an emotional news conference in Chicago, Payton disclosed that he had primary sclerosing cholangitis, a rare condition in which the bile ducts are blocked. He said doctors had told him he would need a liver transplant within two years. A week later, after further tests, doctors told him he needed a transplant by the end of 1999. He was said to have been on a waiting list with 12,000 others at the Mayo Clinic. Dr. Gores said yesterday that Payton had received chemotherapy and radiation treatment after the cancer was diagnosed but it had progressed to where ''transplantation was no longer a viable option.''

The fact that Payton had cancer was not made public before his death.

Playing from 1975 through 1987 for the Bears, who endured many mediocre seasons before becoming winners, he set a host of National Football League records. He retired with career marks for rushing yards (16,726) and attempts (3,838), combined yards (21,803 rushing, receiving and returning), 100-yard rushing games (77) and 1,000-yard rushing seasons (10). He was voted the league's most valuable player in 1977 and was voted nine times to the Pro Bowl.

Marcus Allen and Emmitt Smith subsequently broke his record of 110 rushing touchdowns. Payton's other career records still stand. The only other player within 3,000 yards of Payton's rushing total is Barry Sanders, who retired from the Detroit Lions before the 1999 season with 15,269 yards.

After Payton's retirement, he turned his full attention to the business career he began during his playing days and earned far more than he ever had on the football field.

Jim Finks, the late general manager of the Bears, once described Payton as ''a complete football player, better than Jim Brown, better than O. J. Simpson.'' Yesterday, Dick Butkus, the Bears' Hall of Fame linebacker who retired two seasons before Payton arrived, called him ''a perfect example of what a true back was supposed to be.'' And Paul Tagliabue, the N.F.L. commissioner, praised Payton as ''an inspiration in everything he did.''

At 5 feet 10 1/2 inches and 204 pounds, Payton was not big for a running back, but his strength and conditioning were legendary. He could bench-press 390 pounds and do leg-press series with more than 700 pounds. He could throw a football 60 yards, punt it 70 yards, kick a field goal of 45 yards and walk across a football field on his hands.

Fred O'Connor, the Bears' former backfield coach, once said, ''The first time I saw Walter Payton in the locker room, I thought God must have taken a chisel and said, 'I'm going to make me a halfback.' ''

One part of Payton's conditioning regimen took on mythic proportions in the football world. For several months before the start of each N.F.L. season, he would run up a steep hill near his Illinois home about 20 times a day. Sometimes he would bring along teammates or local college players. ''I enjoy seeing them try it once or twice, and then vomit,'' he said.

Although he routinely carried the ball 20 or more times a game and was known as one of the best blockers among running backs, Payton was never seriously injured. In his 13 professional seasons, he missed only one game, during his rookie season when he had an injured ankle. Missing that game grated on him for years, and he blamed an assistant coach for not letting him play.

Mike Ditka, who coached Payton during the last half of his pro career, called him ''the very best football player I've ever seen, period, at any position.''

Payton was given his nickname, Sweetness, by teammates in the Senior Bowl college all-star game. He was unsure whether it referred to his sweet movements on the field or to the sweet, sincere personality and high-pitched voice.

Known as a selfless team leader, Payton would hand the ball to an offensive lineman after scoring a touchdown because, he said, ''They're the ones who do all the work.''

Payton was elected to the Pro Football Hall of Fame in 1993, the first year he was eligible, and to the College Football Hall of Fame in 1996.

For his induction ceremony at the Pro Hall of Fame, he chose as his presenter his son, Jarrett, then 12 years old. In his speech, Jarrett said, ''Not only is my dad an exceptional athlete, he's my biggest role model and best friend.'' The son, now a freshman running back and kick returner at the University of Miami, returned home Wednesday to be with his father.

Even at 33 and in his final season, Payton had undiminished devotion to football. ''This is something I want to do,'' he said, ''something I love to do. Money isn't everything. Do you get married because of money? Do you have kids because of money?''

Payton seemed almost hyperactive, never wasting a moment. Paul Zimmerman wrote in Sports Illustrated: ''You get a feeling of electricity. Sparks seem to shoot from him.''

The sparks flew when Payton carried the ball. He was aggressive rather than graceful. He ran with power and would never go out of bounds if he could pound out an extra yard or two. He excused the way he punished tacklers by saying: ''What about the pain they've dealt out to me? Pain is expected in this game.''

Jack Youngblood, a defensive end for the Los Angeles Rams, said: ''I remember a block he threw on me once and I thought he opened a hole in my sternum, he hit me so hard. I said, 'Walter, what are you doing?' He said, 'You were in the way.' ''

If Payton did not run over defenders, he disposed of them with a stiff-arm to the head. Everson Walls, a cornerback who spent most of his career with the Dallas Cowboys, recalled: ''I caught his stiff-arm once, under my chin. It bent my head back, and where your head goes, your body goes.''

Payton did not apologize for his aggressiveness.

''I guess my running comes by instinct,'' he said.

Walter Jerry Payton was born July 25, 1954, in Columbia, Miss. He was one of three children of Peter Payton, a factory worker, and his wife, Alyne.

As a youngster, Walter was a good student more interested in music than sports. In ninth grade, he was a long jumper on the track team and played drums in the school band at his segregated high school. After school, he played and sang in jazz-rock combos.

He avoided football, partly to avoid competing with his older brother, Eddie, a star running back. After Eddie graduated, the coach asked Walter, a sophomore, to try out for the team. He agreed only after the coach allowed him to stay in the band.

The first time Payton carried the ball, he gained 65 yards. His school merged with an all-white school in his junior year, and during his final two years he scored in every game and made the all-state team. He also long-jumped 22 feet 11 1/4 inches, averaged 18 points a game for the basketball team, played baseball and was a drummer in the band.

Although major colleges sought him, he followed his brother to Jackson State College, a historically black college that is now Jackson State University. As a junior he led the nation in scoring with 160 points, and his career total of 464 points set what was then a National Collegiate Athletic Association record.

At 20, he had a bachelor's degree in communications, earned in three and a half years, and began work on a master's in education for the deaf. In ''Sweetness,'' his autobiography, he wrote that he had studied hard ''to help dispel the myth that athletes in general and black athletes in particular don't have to work to get their diplomas and that they don't learn anything anyway.''

In the N.F.L.'s 1975 draft, with the fourth choice over all, the Bears picked Payton. He promised, ''When I get through with Chicago, they're going to love me.''

He was right. The Bears gave him a $126,000 signing bonus, a team record, and in his first season he led the league in kickoff returns. In 1977, he had that 275-yard rushing game against Minnesota, and at 23 he was voted the N.F.L.'s most valuable player, at the time the youngest ever to win the award.

That led to a three-year contract, starting in 1978, with annual salaries of $400,000, $425,000 and $450,000, dizzying numbers then. After the 1983 season, he signed the N.F.L.'s most lucrative contract to that point, with an annuity that guaranteed him $240,000 a year for life.

In 1984, the Bears, who had become a winning team, reached their conference championship game. The next season, after going 15-1, they routed the New England Patriots, 46-10, in Super Bowl XX, their first N.F.L. championship since 1963. Payton rushed 22 times for 61 yards and was often used as a decoy. ''When they're keying on you, you can't mind,'' he said.

Payton's last season was 1987, when he had to share his position with Neal Anderson. Still, he did not act his age of 33. On the first day of training camp, when others were dehydrated, he took extra sprints.

But later that season, he broke down in the locker room and cried. He realized it was time to go.

''Sometimes,'' he said, ''I feel I'm the problem here. A lot of times I feel I don't even belong here. These are feelings I've never felt before. It's hard.''

He enjoyed football, but when it ended, it ended.

''That doesn't mean I miss it,'' he said. ''It was a job. It was work.''

He enjoyed the business career that followed, too. During his playing days, he had founded Walter Payton Inc., with offices now in Hoffman Estates, outside Chicago. After retirement, he devoted more time to the company's investments, mainly in real estate, restaurants, timberland, travel agencies and nursing homes.

He began to race cars and boats. He owned an auto-racing team. He was a partner in as many as 20 restaurants and nightclubs. He was involved in promotions, endorsements and charities and was a highly paid motivational speaker.

He became a member of the Bears' board of directors. He held a 15 percent interest in a group that bid in 1993 for an N.F.L. expansion franchise for St. Louis. That would have made him the league's first black owner. The franchises went instead to Jacksonville and Carolina.

He also found more time to spend with his family. He is survived by his wife, Connie; his son, Jarrett; his daughter, Brittney; and his mother, Alyne.

In retirement, he had no regrets. ''What are you going to do, erect a statue that says, 'May he rest in peace'?'' he asked. ''If you could go back and change things, you might not be the person you are right now. And I'm pleased with the way I am now. There were times it was frustrating and there were times it was jubilant. Over all, it's been a lot of fun.''

Walter J Payton