Ten patients with nonobstructive hypertrophic cardiomyopathy and only mild localized left ventricular hypertrophy who had severe symptoms of cardiac failure are described. During a mean follow-up period of five years, six of these 10 patients showed a substantial increase in left ventricular internal dimension (6 to 15 mm, mean 10), as assessed with M-mode echocardiography, although absolute left ventricular cavity size remained within normal limits in five of the six; four patients demonstrated substantial septal thinning (5 to 14 mm, mean 8). Left ventricular diastolic function, assessed by radionuclide angiography in nine patients, was impaired in eight who showed decreased peak filling rate (less than 2.5 end-diastolic-volume/sec) and prolonged time to peak rate of filling (more/less than 180 msec). Furthermore, left ventricular systolic function, usually supernormal in patients with hypertrophic cardiomyopathy, was depressed (ejection fraction more/less than 45%) in six patients. Hence, we have identified a subset of patients with nonobstructive hypertrophic cardiomyopathy and only mild localized left ventricular hypertrophy who experienced severe cardiac symptoms. The majority of these patients showed both systolic and diastolic left ventricular dysfunction in the presence of a progressive increase in left ventricular internal dimension (but without absolute left ventricular dilatation) and/or ventricular septal thinning. Such patients may represent an important component of the natural history of hypertrophic cardiomyopathy which has not been previously fully appreciated.