The purpose of this study was to determine if the thickened cardiac walls of patients with hypertrophic cardiomyopathy (HC) are due to increased size or number of myocytes or increased amounts of fibrous tissue. Eight patients, aged 18 to 42 years, who died from complications of HC and 8 age-matched control subjects without heart disease were studied. A 1.5-cm3 block of tissue was removed from the left ventricular free wall, right ventricular free wall and ventricular septum (VS). Each region of each wall was evaluated for fibrous tissue by point counting; cell diameter was measured using an ocular micrometer disc. Cell layers were counted across the walls. The results revealed that increased cell size, cell layers and fibrous tissue are characteristic of HC, but only in the VS are all 3 significantly increased. The fibrous tissue was most extensive in the VS (19 +/- 9%), but it was more extensive than in the control subjects in all 3 walls. Cell diameters were largest in the layers closest to the left ventricular cavity.