The calculation of body composition using bioelectrical impedance analysis in sick and hospitalized patients is hampered due to altered hydration state. We wanted to investigate how disease-related malnutrition assessed by the Subjective Global Assessment (SGA) is reflected in the bioelectrical impedance vector analysis. Patients with benign gastrointestinal disease (n 242) were entered in the study. Nutritional status was assessed by SGA. Arm muscle and fat area were estimated by anthropometry, muscle function was determined by hand grip strength. Whole body impedance measurements were made at 50 kHz. Ninety-eight patients were considered well nourished (SGA A), ninety-four were classified moderately malnourished (SGA B) and fifty patients were classified severely malnourished (SGA C) according to the SGA. The mean vector was significantly displaced between SGA C and SGA A and B, showing comparable resistance with a significantly reduced reactance, indicating comparable hydration but loss of dielectrical mass of soft tissues. This distinctive vector migration was not seen when studying the patients grouped according to BMI. In conclusion, disease-related malnutrition as assessed by the SGA is associated with a distinctive bioelectrical vector migration, implying that abnormal tissue structure and not reduced body mass only occurs in disease-related malnutrition. These disturbances are not seen in underweight according to BMI. Bioelectrical impedance vector analysis appears to be an attractive tool to identify disease-related malnutrition and to monitor nutritional intervention.