The ventricular septal myotomy-myectomy operation for obstructive hypertrophic cardiomyopathy requires intimate knowledge of ventricular septal anatomy for successful outcome. Often routine echocardiography can not provide reliable information regarding septal morphology because of technical limitations. Therefore, we utilized a sterilizable 5MHz two-dimensional echo transducer for intraoperative studies in 9 patients with hypertrophic cardiomyopathy; echos were performed by placing the transducer directly on the anterior surface of right ventricle. In 6 of 9 patients, preoperative transthoracic M-mode and 2-dimensional echos did not provide definitive assessment of ventricular septal thickness. In each of these patients, measurements of ventricular septal thickness varied by 5-11 mm (average 7) due to suboptimal visualization of ventricular septal endocardial surfaces. However, in each patient intraoperative echocardiography provided a clear definition of endocardial borders and more accurate definition of ventricular septal thickness. In one patient transthoracic echo underestimated ventricular septal thickness by 5mm; in the remaining 2 patients ventricular septal thickness by transthoracic and intraoperative echo were similar. Intraoperative echocardiography performed after resection of ventricular septal muscle also provided direct morphologic assessment of the depth, length and width of the ventricular septal myotomy-myectomy resection. Therefore, our initial experience indicates that intraoperative two-dimensional echocardiography is a valuable aid to the surgeon performing the ventricular septal myotomy-myectomy operation.