The natural courses of 82 patients with hypertrophic cardiomyopathy (HCM) were investigated in follow-up periods of over 10 years (mean follow-up: 11.7 yrs.). Twelve patients had obstructive, 30 non-obstructive and 40 apical HCM. There were 76 males and 6 females. The mean age at the initial diagnosis was 48 years. All patients underwent cardiac catheterization and left ventriculography, and two-dimensional echocardiography was conducted in some patients. To determine the clinical features influencing the prognosis, their serial laboratory and clinical data were reviewed and analyzed. Five patients died of non-cardiac causes. Only one died suddenly. Congestive heart failure developed in 3 patients, 2 with obstructive and one with non-obstructive HCM. Two patients had cavity dilatation and deteriorated ventricular function, and finally exhibited dilated cardiomyopathy-like features. Characteristically, in this deteriorating group, a decrease in the QRS voltage and an abnormal Q wave gradually developed without clinical evidence of myocardial infarction. Two patients with apical HCM in this group had decreases in their QRS voltages and in the depths of their giant negative T waves. They developed apical left ventricular asynergy without myocardial infarction or congestive heart failure. There were no specific clinical or laboratory parameters predictive of sudden death. Atrial fibrillation occurred in 9 patients, resulting in 3 cases of cerebral infarction, one myocardial infarction, and one congestive heart failure. One patient received pacemaker implantation because of the sick sinus syndrome. Three had acute myocardial infarction. Aortocoronary bypass grafting was performed in 3 patients. These results indicate that the good long-term life prognosis of HCM can be expected in all types of HCM. Among the 82 patients, only one died suddenly. However, since the natural history of HCM can take a variety of courses, careful observation is necessary.