The aim of this study was to determine if changes in the shape of the third metacarpal bone (MC3) during the first preparation for racing in young Thoroughbred racehorses could be measured radiographically. Lateral radiographs of each MC3 were taken when the horses commenced training, every 2 weeks for 10 weeks, and again at 6 months after the start of training. The width of the dorsal cortex (DC), palmar cortex (PC), and medulla (M) were measured at a position 2.5 cm distal to the nutrient foramen before the subject of each radiograph was identified. The following ratios were calculated: z=DC/M; RI = [(DC+PC)/M]x[DC/PC]. The results showed that the shape of the MC3 changed during training for racing and that these changes may be measured on standard lateral radiographs. The dorsal cortex became proportionately thicker than the rest of the bone dimensions in the dorsopalmar plane. The proportionate increases in dorsal cortex dimensions were greater in horses that were recorded as shin sore during racing than in horses that were racing with no recorded shin soreness.