A runner has completed 80 km in a 12-hour run. Prior and shortly after the run, fat and skeletal muscle mass were determined non invasively both by the bioelectrical impedance analysis and the classic skin fold method. In addition, blood and urine samples were taken in order to assess fluid balance. By applying the bioelectrical impedance analysis, the runner has increased body mass by 1.5 kg, fat-free body mass by 4.2 kg and muscle mass by 1.0 kg, whereas fat mass decreased by 4.4 kg. Since body water increased by 4.9 l, the determination of haematocrit, haemoglobin and sodium showed a haemodilution and the specific gravity of urine indicated no dehydration, we assume a substantial decrease of subcutaneous adipose tissue for energy production and intracellular oedemas. The difference between determining fat mass with the skin fold method or with the bioelectrical impedance analysis is discussed.