Bioelectrical impedance analysis (BIA) is a widely used method for calculating body compartments in healthy subjects and chronically ill patients. The resistance (R) – which is correlated with total body water – and the reactance (Xc) – depending on the capacitance of cell membranes – are measured by BIA. The phase angle α, a mathematical relation between R and Xc, has qualified as a prognostic marker in several severe diseases [1]. We aimed at monitoring body composition during the clinical course of ICU patients and to investigate the impact of illness severity on BIA.