Determination of right ventricular ejection fraction (RVEF) provides information about global right ventricular function, which may be important for the management of patients with various heart diseases. Right ventricular ejection fraction can be determined by new thermodilution techniques using fast-response thermistors. To evaluate the validity of these methods, thermodilution measurements were compared with biplane cineventriculography in 22 patients undergoing cardiac catheterization. In all patients standard deviation of RVEF was below 5%. Mean RVEF, determined by thermodilution, was 52% +/- 9%, ranging from 32% to 71% and correlated significantly with the results of angiography (RVEF: 52% +/- 9%) (r = 0.80, SEE +/- 5%, n = 22, p less than 0.001). Correlation was good especially in patients with small right ventricles (less than 160 ml) (r = 0.91, SEE +/- 5%, n = 13, p less than 0.001), lower heart rates (less than 65/min) (r = 0.84, SEE = +/- 6%, n = 12, p less than 0.001) and cardiac output below 5.51/min (r = 0.88, SEE +/- 6%, n = 11, p less than 0.001). Thus, if valid catheter placement is possible, right ventricular ejection fraction can be determined by thermodilution technique with good reproducibility and sufficient accuracy compared to biplane angio. Validation of this method in larger patient populations with various heart diseases is necessary.