Abstract Objective—To evaluate changes over time in echocardiographic measurements in young Standardbred racehorses undergoing training and racing and determine whether there was any relationship between cardiac dimensions and racing performance. Design—Longitudinal observational study. Animals—103 horses. Procedure—2-dimensional and M-mode echocardiography was performed 4 times at 6-month intervals. Results—Significant cardiac enlargement took place during the study period as indicated by increases in left ventricular internal diameter in diastole (LVIDd), estimated left ventricular muscle mass (LV mass), and mean wall thickness attributable to eccentric left ventricular hypertrophy. Estimated body weight was positively correlated with left ventricular size, and males had significantly larger LVIDd and LV mass than did females. Horses that were racing regularly had larger LVIDd and LV mass than did unraced horses. A significant relationship between left ventricular size and racing performance was observed. The relationship was strongest at the time of the fourth examination. Conclusions and Clinical Relevance—Results suggest that age must be taken into account when interpreting results of echocardiography in young Standardbred racehorses because significant cardiac enlargement takes place with age and training. A larger heart was found in horses that were racing, and size of the heart was correlated with athletic performance of the horse. (J Am Vet Med Assoc 2005;226: 1881–1887)