Echocardiographic and histological studies were performed for four patients with hypertrophic cardiomyopathy (HCM) with right-sided obstruction (group A) and for five HCM patients without right-sided obstruction (group B). The clinical diagnosis of HCM was established by cardiac catheterization and right and left ventriculography for all nine patients, and confirmed by necropsy in one patient in group A. M-mode and cross-sectional echocardiography were performed in all. Endomyocardial biopsy of the right ventricular septum was performed for all four patients in group A and for three patients in group B. M-mode echocardiography demonstrated no characteristic features suggesting right intraventricular obstruction, but cross-sectional echocardiograms, especially with short-axis views, reliably indicated conspicuous rightward septal protrusions in right-sided obstructive HCM. Histological changes in the right ventricular septum consisting of fibrosis of the interstitium and disarray of myocardial fibers were more striking in patients with HCM with right-sided obstruction than those without such obstruction.