Patients with idiopathic hypertrophic subaortic stenosis (IHSS) have supernormal left ventricular systolic function at rest. Septal myotomy/myectomy reduces outflow obstruction and symptoms. However, it has been proposed that operation causes myocardial damage, with consequent reduction in left ventricular function and potentially deleterious long-term consequences, despite relief of symptoms. To evaluate the effect of myotomy and myectomy on left ventricular function, we employed noninvasive radionuclide cineangiography at rest and during maximal exercise in 21 consecutive patients with IHSS, before and 6 months after operation. In 30 normal subjects, the ejection fraction (EF) was 57 ± 2% at rest and 71 ± 2% during exercise. Before operation in patients with IHSS, the EF was supernormal at rest (76 ± 2%, p < 0.001), and normal during exercise (71 ± 3%, NS). All patients improved from NYHA functional class III-IV preoperatively to class I-II postoperatively; the maximal provocable gradient invariably diminished (average, 100 ± 8 mm Hg preop, 33 ± 9 mm Hg postop, p < 0.001). Postoperatively, the EF at rest was 67 ± 2%, still supernormal, but less than that noted preoperatively (p < 0.001). During exercise, the EF was 66 ± 4%, only mildly diminished (p < 0.01) from that preoperatively. We conclude that muscle resection during myotomy and myectomy usually does not cause any important impairment of global left ventricular function at rest or during exercise stress.