OBJECTIVES: 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. DESIGN: Observational, clinical study. SETTING: A 17-bed, surgical intensive care unit in a university hospital. PATIENTS: Twenty-six patients before and after open-heart surgery with cardiopulmonary bypass. INTERVENTIONS: None. MEASUREMENTS AND MAIN 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. CONCLUSION: Cardiac surgery produced a significant decrease in segmental trunk BIA, reflecting fluid accumulation at the trunk level.