LOUISVILLE, Ky. — Big Brown backed up his trainer's boasts with an explosive finishing kick and won the Kentucky Derby today, a commanding victory turned somber by the fatal breakdown of the filly Eight Belles on horse racing's biggest day.

The cheers for the winner's 4¾-length victory were cut short when Eight Belles, the runner-up, collapsed while galloping out near the second turn. She broke her two front ankles and was euthanized on the track minutes later.

"When we passed the wire I stood up. She started galloping funny. I tried to pull her up. That's when she went down," said her distraught jockey, Gabriel Saez.

Eight Belles was attempting to become the fourth filly to win the Derby.

The unbeaten Big Brown's start from the outside post did little to hamper his charge when the 20-horse field turned for home at Churchill Downs. Under the urging of jockey Kent Desormeaux, the 2-1 favorite cruised to an easy victory to become the seventh unbeaten Derby winner with his fourth consecutive win. The last one was Barbaro in 2006.

That wasn't the only reason thoughts of Barbaro were hard to ignore on this Derby Day.

The breakdown brought back memories of the 2006 Preakness, where Barbaro shattered his right rear leg just after the start. The colt was euthanized months later, after developing laminitis from the catastrophic injuries.

The tragedy undoubtedly drained some of the joy from Big Brown's victory, which sends him to the Preakness in two weeks as the only 3-year-old with a chance to become the first Triple Crown champion since Affirmed in 1978.

"We're ready to roll," Big Brown's confident trainer Rick Dutrow Jr. said.

All week, Dutrow told the world he had the best horse in the field — and the big bay colt justified every accolade tossed his way.

"I can't describe the feeling that all of us have right now," he said.

The colt became the first Derby winner since Regret in 1915 to have raced only three times previously. He is only the third in 60 years to win after racing in just two Derby preps — Sunny's Halo in 1983 and Street Sense last year were the others.

In addition, Big Brown became the second winner to start from the No. 20 post. The gelding Clyde Van Dusen did it in 1929.

Big Brown covered the 1¼ miles in 2:01.82 in front of the second-largest crowd in Derby history at 157,770. He paid $6.80, $5 and $4.80.

Eight Belles paid $10.60 and $6.40, and Denis of Cork, at odds of 27-1, returned $11.60.

Dr. Larry Bramlage, the Derby's on-call veterinarian, said the filly's injuries were too severe to even attempt to move her off the track.

"She didn't have a front leg to stand on to be splinted and hauled off in the ambulance, so she was euthanized," Bramlage said.

Larry Jones, who trains Eight Belles paid tribute to his filly, who was scratched out of Friday's Kentucky Oaks for a chance to take on the boys.

"There was no way to save her. She couldn't stand," Jones said. "She ran an incredible race. She ran the race of her life."

Tale of Ekati was fourth, followed by Recapturetheglory, Colonel John, Anak Nakal, Pyro, Cowboy Cal, Z Fortune, Smooth Air, Visionaire, Court Vision, Z Humor, Cool Coal Man, Bob Black Jack, Gayego, Big Truck, Adriano and Monba.

The colt earned $1,451,800 for the win and boosted his earnings to $2,114,500 for owners IEAH Stables and Paul Pompa Jr. Pompa, who named Big Brown in honor of United Parcel Service, a client of his trucking business, sold a 75 percent interest in the colt to IEAH for about $3 million after his first race.

Desormeaux won the Derby for the third time, having won aboard Real Quiet in 1998 and Fusaichi Pegasus in 2000. Only three other riders have won more — Eddie Arcaro, Bill Hartack and Bill Shoemaker.

"It was smooth sailing all the way," Desormeau said. "The horse was very comfortable."

Big Brown was the third favorite to win in the past five years. Smarty Jones won in 2004 and Street Sense won last year.

All eyes were on Big Brown at the start. Dutrow called his colt the fastest of all and he proved it when Desormeaux gunned him close the lead on the mad dash to the first turn. Desormeaux did a masterful job of keeping Big Brown free and clear of any traffic issues.

As the field headed into the backstretch, Big Brown was in sixth place and just waiting for Desormeaux's signal to make his move. It came around the far turn, and Big Brown took the lead at the top of the stretch and was never challenged to the wire.

"I don't even know what we just did," Dutrow said. "I can't express my feelings, only that it was one of the most incredible feelings I ever had, and I can't wait to feel it again."