The forelimb superficial digital flexor (SDF) tendons of 6 Thoroughbreds were examined clinically and ultrasonographically during the first 4 months of race training. Sonograms were interpreted clinically and by use of computer-aided analysis. Tendon tissue from all horses was examined histologically at the end of the study. Computer-aided analysis of sonograms of the SDF tendons revealed trends toward an increase in mean cross-sectional area and a decrease in mean echogenicity over time with training. An inverse relation was found between increase in cross-sectional area and decrease in mean echogenicity over time in training. Two of the trained horses developed clinical signs of mild SDF tendonitis. Ultrasonography revealed an increase in cross-sectional area and decrease in mean echogenicity of clinically affected areas of the SDF tendons of 1 horse, compared with changes observed prior to the onset of tendonitis (these changes were not statistically significant). Blood vessels and lymphatics supplying the clinically and ultrasonographically affected tendon sites were large and thick-walled. These changes were not observed in the tendons of the other horses at the end of the study. The authors conclude that equine SDF tendons adapt to the early months of race training by increasing in size and decreasing in echogenicity, as determined by ultrasonography.