To clarify the cause of myocardial hypertrophy in ischemic heart disease, the relation between the extent of fibrosis and myocyte diameter was examined in patients with cardiac hypertrophy of unknown etiology except for coronary sclerosis. In 79 unselected cases, the heart weight tended to be higher in patients with severe coronary stenosis and fibrosis. In a morphometric study of 33 additional hearts of patients without a clinical history of hypertension, valvular disease or diabetes mellitus, 15 of which had anterior infarction, a positive correlation (r = 0.63) was observed between myocyte diameter and the percent area of fibrosis in the anterior wall of the left ventricle. In the heart of patients with severe coronary stenosis (more than 75% luminal narrowing), the regression coefficient was 0.83. The hearts of 8 patients with nontransmural infarction showed a strong correlation between myocyte diameter and fibrosis, compared with the hearts of 7 patients with transmural infarction. In most cases, the main mechanism of hypertrophy in ischemic heart disease was considered to be compensatory hypertrophy for existence of myocardial fibrosis.