Natural history of hypertrophic cardiomyopathy in the elderly. The prognosis of patients diagnosed as having hypertrophic cardiomyopathy at advanced age has not been well defined. This study details follow-up information obtained for 95 patients initially diagnosed as having hypertrophic cardiomyopathy at age greater than or equal to 65 years. Seventy-five percent of patients were symptomatic, as defined by the presence of chest pain, dyspnea or syncope, and the mean ventricular septal thickness was 20 mm. The median duration of follow-up study was 4.2 years. The survival rate at 1 and 5 years was 95% and 76%, respectively, which was not significantly different from that an age- and gender-matched control group. Of patients presenting with New York Heart Association functional class I or II dyspnea, only 18% progressed to class III or IV during the follow-up period. However, patients presenting with class III dyspnea had a 1 year mortality rate of 36%, significantly higher than that of control subjects (p less than 0.003). Of the echocardiographic variables, indexed left atrial size was most strongly associated with reduced survival (p less than 0.008). These results suggest that the prognosis of elderly patients with hypertrophic cardiomyopathy is generally favorable. Certain clinical and echocardiographic variables appear to be of use in identifying patients with a less favorable prognosis. 