We studied five cases of atrial fibrillation in the elderly showing severely decreased exercise tolerance (NYHA III) and normal left ventricular function. There were no signs of classical tricuspid regurgitation. Both atria were enlarged and right ventricular sizes were shown by two-dimensional echocardiography to have mildly increased. Two-dimensional Doppler echocardiography revealed severe tricupid regurgitation but at a maximal velocity less than 2.3 m/sec, meaning that right ventricular pressure was not increased. Cardiac output (CO) and circulating blood volume (BV), measured by radiocardiography showed CO to have decreased and BV to have increased. No intracardiac shunt was detected. The cause of this decreased exercise tolerance in the group in our study is thought to be right ventricular dysfunction due to the aging process. This also occurs in a similar pattern in younger people who have arrhythmogenic right ventricular dysplasia (ARVD), Uhl's disease, or right ventricular dilated cardiomyopathy.