Forty-seven consecutive patients aged 1-38 years underwent operation for hypertrophic obstructive cardiomyopathy (HOCM) between 1972 and 1986. Isolated myectomy-myotomy was performed in 43 patients, three patients underwent myectomy and concomitant aortic valve repair, and one patient underwent concomitant mitral valve replacement. The peak systolic pressure gradient from the left ventricle to the aorta decreased from 70 +/- 33 mm Hg (mean +/- SEM) preoperatively to 10 +/- 15 mm Hg immediately after repair (p less than 0.001). Moderate or severe mitral insufficiency was identified in 16 patients preoperatively and was corrected by myectomy alone in 15. There was no operative mortality; two late deaths occurred during follow-up (median, 5 years; maximum, 16 years) for estimated 5- and 10-year survivals of 97 +/- 2% and 88 +/- 10%, respectively. Reoperation was required for aortic valve replacement (n = 2), remyectomy (n = 2), and permanent pacemaker implantation (n = 3). Preoperative symptoms were relieved in 24 of 29 (83%) patients with dyspnea, in 18 of 19 (95%) with angina, and in six of 10 (60%) with syncope. These results support myectomy-myotomy for symptomatic children and young adults with HOCM. Also, it appears that late survival after myectomy-myotomy in these young patients may be improved over that observed in historical controls treated with medications alone; operation should be considered for asymptomatic children and adults less than 40 years of age with large (greater than 80 mm Hg) left ventricular outflow gradients.