In this study we used Doppler echocardiography to examine changes in ventricular diastolic function in neonates. Echocardiograms were performed in 25 healthy neonates at 1-6 days (2.7+/-1.6 days) and at 25-35 days (29.4+/-3.3 days) after birth. Ventricular inflow velocity curves were recorded using the pulsed-Doppler method. Peak velocities (peak E, A). mean velocities (mean E, A), times (time E, A), and diastolic velocity-time integrals (area E, A) were measured during both early ventricular filling and atrial systole. The acceleration time of the wave during early ventricular filling was also measured. Peak E and A and mean E and A in both ventricles increased at 1 month after birth. Acceleration times in both ventricles were shortened. The ratio of area E to A in both ventricles and of peak E to A in the left ventricle did not change at 1 month after birth. Only the ratio of peak E to A in the right ventricle increased. The ratio of peak E to (area E + area A), which is thought to be a load-independent index, increased in both ventricles. The authors concluded that ventricular relaxation appears to increase in both ventricles, 1 month after birth, and that changes in diastolic function occur even in neonates.