In many patients, right ventricular (RV) dysfunction is due to RV infarction. In some patients, however, RV dysfunction may be secondary to the left ventricular (LV) dysfunction. In order to clarify the influence of poor LV function on RV function, RV ejection fraction (EF) was evaluated serially by thermodilution techniques (REF-1, Edwards Laboratories) in patients with LVEF less than 40% who underwent aorto-coronary bypass surgery with uneventful postoperative course. The patients were divided into four groups depending on the site of LV infarction; anterior (n = 13), inferior (n = 8), anterior + inferior (n = 16), and no infarction (n = 4). Control (n = 11) consisted of the patients with LVEF more than 60% and with no significant stenosis of the right coronary artery. Cardiac index, intracardiac pressures and amount of catecholamine used during postoperative course showed no significant differences among the groups including control. However, the groups except for the group of anterior infarction showed significantly low pre- and postoperative RVEF compared with control. Only the group of anterior infarction had almost normal RVEF. These results mean that RV dysfunction associated with inferior infarction remain long afterwards and that low LVEF due to anterior infarction caused little effect on RV function.