The accuracy and limitations of intraoperative two-dimensional (2-D) and color Doppler flow mapping transesophageal echocardiography (TEE) of ventricular septal defect (VSD), before and after cardiopulmonary bypass, were analyzed in 62 children. Twenty-one patients had an isolated VSD, and 41 had a VSD plus additional cardiac anomalies. Two-dimensional and color Doppler flow mapping TEE were performed with a miniaturized 5-MHz single (transverse) plane transducer in the 51 of 62 patients weighing less than 20 kg. The remaining 11 were monitored using a single plane adult probe (n = 4) and a biplane (transverse plus longitudinal) probe (N = 7). Prebypass TEE provided a correct diagnosis in 57 of 62 cases (92%) and corrected an erroneous preoperative transthoracic echocardiographic diagnosis in three of 62 cases (5%). Single plane TEE diagnosis was erroneous in five patients: four with doubly-committed subarterial VSD and one with multiple small apical muscular defects and pulmonary hypertension. Biplane TEE (transverse longitudinal) provided clear and complete imaging of the right ventricular outflow tract in all seven cases in whom it was used. Postbypass TEE showed absence of a hemodynamically significant residual VSD in 30 of 40 patients (95%) who underwent VSD patch closure, prospectively identified two of 40 with significant residual VSD, and accurately measured the color Doppler jet width of all residual VSDs. We conclude that hemodynamically significant VSDs can be identified immediately after cardiopulmonary bypass based on the width of the residual VSD color Doppler flow map jet. Therefore, 2-D and color Doppler flow mapping TEE provide an accurate diagnosis in most cases of VSD but may miss doubly-committed subarterial and apical muscular VSD unless biplane TEE is used.