In children with a ventricular septal defect, transseptal blood flow has been demonstrated angiographically to be bidirectional in all but the smallest defects. To investigate this phenomenon noninvasively, two-dimensional Doppler color flow echocardiography was used in 77 patients (aged 1 day to 15.5 years, mean 24 months). During isovolumetric contraction, flow was seen from the left to the right ventricle in 82 (98%) of 84 studies. During isovolumetric relaxation, right to left flow was noted in 72 (96%) of 75 studies. During early diastole in patients with an isolated defect, flow was initially from the right to the left ventricle and subsequently reversed to become left to right in 29 (91%) of 32 studies. In patients with concomitant right ventricular volume overload, flow across the ventricular defect was from the right to the left ventricle throughout diastole in 30 (86%) of 35 studies. In each of four patients with d-transposition of the great vessels and each of two patients with 1-transposition of the great vessels, blood flow was from the morphologic left ventricle to the morphologic right ventricle during isovolumetric contraction and from the morphologic right ventricle to the morphologic left ventricle during isovolumetric relaxation. These results demonstrate 1) the complex nature of intracardiac shunting in children with ventricular septal defect as previously shown by angiography; 2) an intrinsic functional difference in the contractile and relaxation properties of the morphologic left and right ventricles; and 3) differences in diastolic blood flow patterns between children with an isolated defect and those with a ventricular septal defect and right ventricular volume overload (p = 0.0001).