The effects of endurance training have been extensively studied in athletes, but longitudinal studies of exercise-induced cardiac changes in normal patients are limited. To assess the effects of 6 months of moderate-intensity aerobic training (1 hour/day, 3 times/week) on normal hearts, 23 sedentary men aged 31.1 ± 3.5 years were studied by standard and tissue Doppler echocardiography. Left ventricular (LV) systolic function was assessed by the ejection fraction and Doppler-measured stroke volume, and diastolic function was assessed by transmitral and pulmonary venous flow. Tissue Doppler systolic (Sm), early (Em), and late (Am) myocardial velocities were obtained at the septal and lateral mitral annulus. After training, there was a 14.5% increase in peak oxygen consumption (p = 0.000002) and a decrease in heart rate (60 ± 7 to 56 ± 8 beats/min, p = 0.01). Septal and posterior wall thickness increased (8.7 ± 1.0 to 9.4 ± 1.3 mm, p = 0.002, and 8.2 ± 0.7 to 8.8 ± 1.1 mm, p = 0.0009, respectively), with a 15% increase in LV mass index (p = 0.0002). LV diameters, stroke volumes, and ejection fractions were unchanged. Mitral inflow showed a decrease in late-wave velocity (p = 0.00004), thus increasing the early (E)/A ratio. Septal and lateral Sm (p = 0.02) and Em velocities (p <0.05) increased after training. In conclusion, the physiologic increase in LV mass in response to regular exercise in healthy young men occurs in parallel with a decrease in atrial contribution to flow. LV function estimated by tissue Doppler is improved despite the lack of changes in standard echocardiographic indexes.