Myocardial Thallium-201 perfusion defects, suggestive of ischemia, are commonly observed among patients with hypertrophic cardiomyopathy, regardless of symptomatic state. Medical therapy is usually reserved for those patients with limiting symptoms, but may be indicated even in asymptomatic patients if such therapy is found to improve or normalize the indices of silent ischmeia often observed in such patients. The current protocol is designed to assess the effects of verapamil on abnormalities of myocardial perfusion and left ventricular diastolic function in asymptomatic and minimally symptomatic patients with hypertrophic cardiomyopathy. The results of this study could alter the clinical management of this very large patient population. Beginning in July 1986, we plan to enroll 20 patients in a randomized trial, comparing verapamil with placebo in the treatment of asymptomatic or minimal asymptomatic patients with hypertrophic cardiomyopathy. Following two weeks of drug therapy, patients will undergo Thallium-201 emission-computed tomography in conjunction with maximal treadmill exercise. Qualitative and quantitative assessments of myocardial perfusion will then be made. The patients will also undergo resting radionuclide cineangiography to evaluate both systolic and diastolic left ventricular function. Following a one week washout period, the patients will then cross over to alternate therapy (either placebo or verapamil) and repeat radionuclide investigations will occur two weeks later. The cardiovascular and systemic effects of either drug will be carefully monitored by close supervision during the course of the study. Should verapamil prove efficacious in improving or normalizing the abnormalities found among these patients, we will then consider a more long-term randomized trial to assess the effects of medical therapy in a much larger patient group.