In 14 critically ill patients in stable cardiopulmonary status, right ventricular ejection fraction (RVEF) was measured by thermodilution technique and by radionuclear (gated first pass) technique. The pulmonary artery catheter was equipped with a fast-response thermistor and an intracardiac ECG monitor. In addition, the proximal lumen ended in a 3-hole port 21 cm from the tip of the catheter to facilitate mixing of the cold bolus above the tricuspid valve. The use of a new algorithm based on an exponential curve analysis of the thermodilution curve limited the variability of RVEF determinations to 7.6%. The correlation between RVEF measured by thermodilution and radionuclear techniques was significant (y = 12.7 +/- 0.49x, r = 0.67, p less than 0.01). However, the values obtained by thermodilution were usually lower, especially for high RVEF. Nevertheless, although some discrepancy was found, thermodilution techniques allow simple, accurate and repetitive bedside measurements of right ventricular volumes in the critically ill.