To evaluate the effect of changes in preload on left ventricular (LV) performance, we used echocardiography to measure end-diastolic dimension, end-systolic dimension, and stroke volume in newborns at 2, 12, 24, and 120 h of age. The stroke volume was calculated by the pulsed Doppler technique. The stroke volume showed the highest level at 2 h of age. The size of the ductus arteriosus correlated with the stroke volume. These results indicated that the increase in stroke volume was related to the increase in LV preload due to the shunt flow volume through the patent ductus arteriosus. M-mode echocardiographic indexes such as end-diastolic dimension, LV end-diastolic volume, and LV ejection fraction did not show any significant changes from 2 to 120 h of age. We conclude that M-mode echocardiographic evaluation of LV performance is unreliable in the early neonatal period. Our data also provide a useful basis for the interpretation of abnormal left ventricular systolic function in the early neonatal period.