Radiographs of all 4 fetlocks of 71 Standardbred racehorses were obtained at 3-month intervals for 1 year. Radiographic findings in the abaxial surface of the proximal sesamoid bones were classified into 3 types according to the severity of lesions, and correlation was made with clinical findings at time of examination. Type-1 lesions (1 or 2 linear defects less than or equal to 1 mm wide) were detected in 55% of horses at the start of training; clinical signs of disease were not manifested, and lesions did not become clinically relevant. Type-2 lesions (3 or more linear defects less than or equal to 1 mm wide) were detected in horses not manifesting clinical signs of disease, and were more frequently observed after 3 months of training. However, 66% of horses affected with diseases of the suspensory apparatus, including superficial flexor tendinitis and suspensory desmitis, also manifested this type of lesion. When lameness was observed, it was associated with the soft tissue problem, and the sesamoid bone changes were considered secondary. Type-3 lesions (wide, abnormally shaped linear defects) were detected in 7 horses at the start of the study; lesions remained in horses throughout the study and were consistently associated with lameness during training. Type-3 lesions were considered clinically relevant and indicative of primary sesamoiditis.