Between 1981 and 1992 a total of 10 patients with hypertrophic cardiomyopathy (HCM) were detected by mass screening for heart disease in Tokyo's Adachi Ward. Four were first grade elementary school children and six were first grade junior high school adolescents. Two-dimensional echocardiography at the initial evaluation revealed asymmetric septal hypertrophy in four patients, diffuse hypertrophy of the left ventricle in five, and poor left ventricular contractility with wall thinning in one (dilated phase). Three of the five patients with diffuse hypertrophy progressed to asymmetric septal hypertrophy during the average 4-year follow-up period. The degree of septal thickness and the left ventricular wall thickness index were significantly less than in those of young adult controls (12 ± 3 versus 21 ± 9 mm, p < 0.05; and 22 ± 4 versus 28 ± 16 mm, p < 0.05, respectively). Right ventricular endomyocardial biopsy specimens obtained from 9 of the 10 patients showed features typical of HCM (e.g., myocyte hypertrophy with myofibril disarray) in five patients and atypical features (mainly interstitial fibrosis with perivascular cell infiltration) in another four. One patient with dilated phase disease died of congestive heart failure 6 months after the initial evaluation. These results indicate that HCM detected during mass screening is a mild form of the disease and may have atypical pathologic features, such as interstitial fibrosis and perivascular cell infiltration, mimicking the sequela of chronic myocarditis.