Thermodilution determined right ventricular ejection fraction (RVEF) and cardiac output (CO) were measured in 48 critically ill patients requiring mechanical ventilation and inotropic and/or vasoactive drugs. The coefficient of variation on CO and RVEF were calculated from triple determinations. The average coefficient of variation based on 551 triple determinations was 12.6% for RVEF (range 2-51%) and 4.9% for CO (range 0-24%). If a 10% coefficient of variation was chosen as acceptable, 95% of the CO measurements were acceptable. The coefficient of variation on RVEF only fulfilled the 10% criteria in 46% of the measurements, but if the accepted level was raised to a 20% coefficient of variation, 90% of the measurements were acceptable. The measurement of RVEF and CO are used for calculation of e.g. right ventricular end diastolic volume (RVEDV). By applying the average coefficient of variation of RVEF and CO, the accumulated error on calculation of RVEDV was found to be divided by 15%- +20% at worst. Before derived parameters such as RVEDV are interpreted or compared with previously obtained values, the accumulated error should be calculated. To ensure the quality of the measurements, our recommendation is always to calculate the coefficient of variation for each triple determination of RVEF and CO.