Background Accurate measurements for determining cardiac index can be obtained while patients are supine in bed at various backrest elevations. It is not clear if these measurements are accurate when patients are in a bedside chair. Objective To determine if cardiac index based on measurements obtained with the patient in a chair is similar to cardiac index based on measurements obtained with the patient in bed. Methods A convenience sample of cardiac surgical patients and a method-comparison design were used to compare cardiac index values based on measurements obtained with patients in 2 different positions: in a chair and in the bed. A standard thermodilution technique was used to measure cardiac output. Measurement of cardiac output in the second position was obtained immediately after measurement in the first position. Positions were randomly assigned. Bias and precision were calculated and graphed with the Bland-Altman method. Differences in cardiac index of 0.50 or more were considered clinically significant. Analysis of variance was used to determine differences between cardiac index values for the 2 positions. Results A total of 27 postoperative cardiac surgical patients were studied. Cardiac index values based on measurements obtained with patients in the 2 different positions did not differ significantly (F1,50 = 0.446; P = .51). The mean difference score (bias) between the 2 positions was −0.07 (precision, 0.30) Conclusions The practice of putting cardiac surgical patients whose hemodynamic status is stable back to bed before obtaining measurements for calculation of cardiac index may not be required for accurate values.