BACKGROUND In the assessment of fluid status after cardiac surgery, we applied bioelectrical impedance analysis (BIA) to measure the total body water (TBW), extracellular fluid (ECF), and intracellular fluid (ICF), and evaluated its validity. METHODS Thirty patients who underwent cardiopulmonary bypass (CPB group) and 19 surgical patients not receiving CPB (non-CPB group). RESULTS The change of BIA values (deltaTBW, deltaECF, deltaICF), body weight and cumulative fluid balance were determined for 120 postoperative hours, and the relationship between BIA values and body weight and fluid balance were evaluated. Postoperative changes in BIA values in the CPB group were compared with those in the non-CPB group. Finally the ECF/ICF ratio and hemodynamic parameters were compared. deltaTBW and deltaECF correlated with changes in body weight and fluid balance, respectively. Especially there was a high correlation in each case although large deviations in the slope of the regression lines were observed. TBW and ECF increased from immediately after operation up to 96 hours (the maximum value was at day 2). On the other hand, ICF decreased from 48 to 72 hours after operation. There were significant high ECF/ICF in the CPB group compared with the non-CPB group from 12 to 72 postoperative hours. We found that ECF/ICF correlated inversely with mean blood pressure, mixed venous oxygen saturation and colloid osmotic pressure, and positively with central venous pressure and pulmonary artery wedge pressure. CONCLUSIONS It was considered that BIA was useful for evaluating the relative changes in TBW and fluid distribution, and ECF/ICF might be a new parameter for abnormal water metabolism after cardiac surgery.