Subaortic obstruction: intraoperative echocardiography as an adjunct to operation. Fourteen patients undergoing operation for subaortic obstruction (membranous obstruction in 11 patients, tunnel obstruction in 2 patients, obstruction due to reduplicated mitral valve tissue in 1 patient) were evaluated by intraoperative epicardial echocardiography. In all 9 patients with "discrete" obstruction who underwent prebypass epicardial echocardiography, the septal and lateral attachments of the lesion were correctly demonstrated. The precise extent of tunnel stenosis was seen in both patients. The lateral attachment of the membrane in 4 patients and multiple extensions in another 2 were identified by the epicardial study (having been missed on precordial echocardiography). The discrete membrane was enucleated in 10 of the 11 patients and was partially resected in 1. One tunnel obstruction was completely relieved; the other was partially relieved. Reduplicated mitral valve tissue in the remaining patient was completely resected. Epicardial imaging after bypass showed remnants of the membrane in 2 patients. Intraoperative Doppler echocardiography and color flow imaging confirmed the absence of clinically significant residual gradients (less than 20 mm Hg) in all but 1 patient with tunnel obstruction. Epicardial imaging provided excellent morphological information about obstructive lesions of the left ventricular outflow tract and enabled immediate assessment of surgical repair. 