Operative results and late follow-up of 63 patients undergoing surgery for hypertrophic obstructive cardiomyopathy (HOCM) were analyzed in relation to the surgical indication. There were 64 postoperative follow-ups because one patient had two myectomy operations. 27 patients without limiting symptoms (group A) were operated on because of a pressure gradient of more than 50 mm Hg. 31 patients (group B) underwent myectomy operation because of limiting symptoms, and 5 patients (group C) required additional surgery because of mitral incompetence or left ventricular aneurysm. Comparison of preoperative clinical and hemodynamic data indicate that patients of group B were in a more advanced stage of disease than patients of group A. Operative mortality was low, with one perioperative death. Operative complications were rare, and after adequate treatment caused no significant morbidity. The left ventricular systolic pressure gradient was relieved in almost every case, with simultaneous reduction of left ventricular enddiastolic pressure. With a few exceptions the patients experienced longstanding improvement of symptoms. Durability of symptomatic improvement and survival were significantly better in patients of group A. Late results in patient group C were unfavorable, mainly because of the additional lesions requiring surgery. Atrial fibrillation and congestive heart failure were relieved by surgery, but recurrence was frequent in late follow-up. Ventricular arrhythmia during postoperative follow-up was common in both group A and group B, without obvious prognostic significance. In the light of these results the operative indications were reconsidered. Surgery is indicated in patients with limiting symptoms not responding to medical therapy, if a left ventricular pressure gradient of more than 50 mm Hg is present. Simultaneous mitral incompetence of severe degree requires primary mitral valve replacement by low profile prostheses. Risk factors, such as reanimation for ventricular fibrillation or a family history of sudden death, may militate in favour of surgical treatment. The operative indication is doubtful in patients with a high left ventricular pressure gradient but without consistent symptoms and without additional risk factors.