The patterns of interventricular septal hypertrophy were analyzed on two-dimensional echocardiograms to differentiate hypertrophic cardiomyopathy (HCM) from hypertensive cardiac hypertrophy. The control group comprised 110 patients without cardiovascular disease who were matched for age and gender with the hypertension group. The hypertension group comprised 110 patients with uncomplicated essential hypertension, and the HCM group comprised 32 patients in whom the wall thickness of the interventricular septum was 12 mm or more at the mid-portion and no underlying heart disease responsible for cardiac hypertrophy was detected. The interventricular septal thickness was measured both at the thickest portion within 15 mm distal to the aortoseptal junction (basal portion: B) and at the mid-portion (M) in the end-diastolic image on the left parasternal long-axis tomograms, and the B/M ratio was calculated in each patient. The B/M ratio was 1.07 +/- 0.16 in the control group, 1.19 +/- 0.18 in the hypertension group, and 0.83 +/- 0.12 in the HCM group. Compared with the control group, the B/M ratio was significantly high in the hypertension group (p < 0.05) and significantly low in the HCM group (p < 0.01). These results indicate that hypertrophy of the interventricular septum is dominant at the basal portion in hypertensive patients but at the mid-portion in patients with HCM.