The hearts of three cases of congestive heart failure with dilated left ventricles developing in patients with symptomatic hypertrophic cardiomyopathy (HCM) were morphologically investigated. The results showed that disproportionate hypertrophy and dilatation of the left ventricles, accompanied by massive fibrosis and myocardial disarray, were present in the three patients. The mean percent area of fibrosis of the left ventricle was 34.7% and 47.4% at the upper third and lower third levels, respectively, and was much more frequently associated with disarray (84.4 +/- 12.3%). Moreover, the fibrosis was most extensive in the lateral wall of the left ventricle, followed by the posterior, anterior, and interventricular walls. The fibrosis was also diffuse regardless of the subendocardial or subepicardial region of the heart. The findings in the present study suggest that the disarray in this particular series of HCM might be responsible for the mechanism of the fibrosis leading to dilatation of the left ventricle.