In 10 of 64 patients with IHSS, operations designed to relieve obstruction were performed. The pertinent preoperative clinical and hemodynamic findings in these patients, the operative methods employed, and the results of operation are presented in the present report. Eight of the patients were males, 2 were females, and at the time of operation their ages ranged from 10 to 54 years. All of the patients were symptomatic and all but 2 were in functional classes III and IV. In each patient left heart catheterization proved the presence of left ventricular outflow obstruction and the peak systolic gradients between the left ventricle and a systemic artery ranged from 68 to 175 mm. Hg (average 110 mm. Hg). The effective orifice areas ranged from 0.44 to 0.99 cm. 2 (average 0.61 cm. 2 ). Pulmonary hypertension was present in 2 patients. In 5 patients simple left ventriculomyotomy was performed, and in the other 5 ventriculomyotomy was combined with resection of a portion of the hypertrophied interventricular septum. One patient, in whom operative pressure measurements indicated satisfactory relief of obstruction, died suddenly on the eighth postoperative day, apparently from arrhythmia. Complete heart block was produced in 2 patients, and these patients have implanted pacemakers. Seven of the 9 surviving patients have been followed for periods of 7 to 51 months, and each has had striking symptomatic improvement. Left heart catheterizations have been carried out postoperatively on one or more occasions in all 7 patients. In 6 no intraventricular pressure gradient was present at rest or during exercise, while in the remaining patient a peak gradient of 24 mm. Hg was evident. The pertinent clinical findings in these 10 patients, the operative methods utilized, and the results of their pre- and postoperative hemodynamic assessments are presented in detail. Consideration is also given to present concepts in the selection of patients for operation, and in the choice of operative techniques.