Fifty-six patients with hypertrophic cardiomyopathy who had echocardiography and cardiac catheterization were studied. Eleven (18%) of 56 patients had right ventricular (RV) thallium-201 visualization at resting scan. RV thallium-201 visualization was observed in the apex or lower half of the RV free wall, which was quite different from that with RV pressure or volume overload. There was no significant correlation between RV thallium-201 visualization and interventricular septal thickness or haemodynamic data. RV systolic pressure and RV dimension were within normal limits whether RV visualization was observed or not. Thus, an abnormal increase of RV mass in hypertrophic cardiomyopathy may lead to RV thallium-201 visualization without RV pressure or volume overload.