Whether exercise-induced increases in left ventricular mass can alter left ventricular diastolic function was evaluated by measuring transmitral flow velocities at rest by Doppler echocardiography in 15 amateur endurance-trained runners and 15 age- and sex-matched sedentary control subjects. Ventricular mass index, end-diastolic volume index and stroke volume index were derived from measurements of M-mode echocardiograms recorded under two-dimensional guidance. All three variables were increased in the runners (p less than 0.01). These findings, plus the lower heart rate at rest (p less than 0.001), were consistent with endurance training. Although the runners had an almost twofold greater myocardial mass index, their peak early diastolic filling velocity and time to peak filling velocity did not differ from those of the sedentary subjects. In runners, the peak filling velocity with atrial systole tended to be lower (p = 0.12), the ratio of peak filling velocity with atrial systole to that of early diastole was less (p less than 0.05) and the percent of stroke volume contributed by atrial systole was less (p less than 0.001). These differences in atrial filling may be related to the lower heart rates at rest in runners. In summary, significant increases in left ventricular mass, when associated with endurance training, do not alter the early diastolic filling of the left ventricle.