Last year's star two-year-old Uncle Mo has been ruled out of Saturday's Kentucky Derby at Churchill Downs (2324 BST) with a stomach problem.
The powerful colt, who was co-favourite with Florida Derby winner Dialed In, was examined by a vet on Thursday.
But his owner Mike Repole withdrew the horse from the first leg of the Triple Crown on Friday.
Dialed In, trained by Nick Zito, will start from stall eight in the now 19-strong field.
Repole will still have representation in the race through the 20-1 shot Stay Thirsty, but his biggest hope was Uncle Mo.
The horse's illness was discovered after he suffered a shock defeat in the Wood Memorial on 9 April and he was put on medication.
Although he showed no signs of discomfort during track work, in his stable his appetite was lower than normal and he lost weight.
If he recovers, he could race in the Preakness Stakes on 21 May.
Aidan O'Brien's hopes of becoming the first European trainer to win the Kentucky Derby, known as the 'Run for the Roses', lie with Master of Hounds.
The horse was narrowly beaten by the Godolphin filly Khawlah in the UAE Derby on Dubai World Cup night and will be ridden for the first time by top American jockey Garrett Gomez, who won the 2007 Kentucky Oaks in the Coolmore colours aboard Rags To Riches.
Jockey Rosie Napravnik will be hoping to become the first female jockey to win the prestigious race when she rides Pants On Fire after the partnership landed the recent Louisiana Derby.
The powerful colt, who was co-favourite with Florida Derby winner Dialed In, was examined by a vet on Thursday.
But his owner Mike Repole withdrew the horse from the first leg of the Triple Crown on Friday.
Dialed In, trained by Nick Zito, will start from stall eight in the now 19-strong field.
Repole will still have representation in the race through the 20-1 shot Stay Thirsty, but his biggest hope was Uncle Mo.
The horse's illness was discovered after he suffered a shock defeat in the Wood Memorial on 9 April and he was put on medication.
Although he showed no signs of discomfort during track work, in his stable his appetite was lower than normal and he lost weight.
If he recovers, he could race in the Preakness Stakes on 21 May.
Aidan O'Brien's hopes of becoming the first European trainer to win the Kentucky Derby, known as the 'Run for the Roses', lie with Master of Hounds.
The horse was narrowly beaten by the Godolphin filly Khawlah in the UAE Derby on Dubai World Cup night and will be ridden for the first time by top American jockey Garrett Gomez, who won the 2007 Kentucky Oaks in the Coolmore colours aboard Rags To Riches.
