OBJECTIVE Bioelectrical impedance analysis (BIA) is based on the physical property of tissues to conduct electrical currents, impedance being inversely related to tissue fluid content. At high frequency, the electrical current flows across both intracellular and extracellular pathways, making the assessment of fat-free mass possible while a low-frequency current flows through the extracellular space. Similarly, segmental BIA may be used to assess segmental body fluid repartition. The aim of this study was to assess fluid accumulation after cardiac surgery by multiple frequency segmental BIA. METHODS Observational, clinical study. METHODS A 17-bed, surgical intensive care unit in a university hospital. METHODS Twenty-six patients before and after open-heart surgery with cardiopulmonary bypass. METHODS None. RESULTS After surgery, fluid accumulation resulted in a decrease in whole-body and segmental bioelectrical impedance in the arm and in the trunk. There was a good correlation between the fluid accumulation measured by fluid balance and by whole-body or segmental impedance changes. The major part (71%) of fluid accumulation occurred in the trunk. Multiple frequency measurements did not indicate a fluid shift between the intra- and extracellular compartments. CONCLUSIONS Cardiac surgery produced a significant decrease in segmental trunk BIA, reflecting fluid accumulation at the trunk level.