In a randomized study, the effects of acute, preoperative hemodilution (HD) (12 mL/kg) on right ventricular function were investigated in coronary artery surgery patients with reduced left ventricular function (ejection fraction < 50%) and significant stenosis of the right coronary artery (RCA). Blood was replaced either by hydroxyethyl starch (HES) solution (ratio 1:1; HD-HES; n = 15) or by Ringer's lactate, (RL) (ratio 2.5:1; HD-RL; n = 15). Fifteen comparable patients without HD served as a control group. Besides commonly measured pressure parameters, right ventricular end-diastolic volume (RVEDV), right ventricular end-systolic volume (RVESV), and right ventricular ejection fraction (RVEF) were measured using a computerized thermodilution technique before and after HD, as well as after extracorporeal circulation (ECC). Right ventricular systolic function, expressed as RVEF, was not changed significantly by HD in any group. Furthermore, right ventricular function of the hemodiluted patients was not impaired by the subsequent ECC procedure. None of the traditionally measured parameters could be correlated significantly to the right ventricular thermodilution variables. It is concluded that moderate HD does not change right ventricular function even when the RCA is significantly stenosed.