This study was performed to determine the prevalence of ulcers in the gastric squamous mucosa in Standardbred racehorses. Observations were performed at monthly intervals between the beginning of their training season and their 1st qualifying race. This study also identified risk factors at different levels of race training. Forty-eight Standardbred racehorses from 3 training stables in Quebec, Canada, were studied. Baseline historical information and gastroscopic findings were recorded at the beginning of the trial, and once a month thereafter, between December 2001 and June 2002, until the horse's 1st qualifying race or the end of the training. Intensity of training ranged from jogging to intensive training just before the 1st race and was assigned an ordinal score. Location of squamous ulcers and their appearance were observed on gastroscopy, and an ordinal score was assigned. Prevalence of squamous ulcers from the 2nd through the 4th month (72-88%) of training remained at a significantly higher level (P = .002 to .04) than at the onset of the study (38%) and was also higher in intensely trained horses than in joggers (93% versus 56%). Moderate or more intensive training increased the odds (odds ratio [OR], 3.39; confidence interval [CI], 1.34-8.56; and OR, 11.4; CI, 3.21-40.5, respectively) of detecting ulcers with higher scores. These odds were also higher in trotters (OR, 2.17; CI, 1.07-4.43) than in pacers and generally increased with the duration of training. Duration of training, training level, and gait type also influenced the number of sites with ulcers in the same way. Ulcers had higher scores along the lesser curvature (LC) and the margo plicatus (MP) areas of the stomach. It was concluded that squamous ulcers appeared early in the training of Standardbred racehorses, that the number of sites affected and the ulcer score are related to the intensity of training, and that trotters are more prone to squamous ulcers than pacers.