Electrical impedance measurements were performed in fasting blood samples and analysed in relation to packed cell volume and calculated intra- and extra-cellular water distribution. The total blood impedance was shown to be strongly dependent on the ratio of intra-cellular water to total water (r = 0.97, P less than 0.0001). In a group of 515 subjects, with a large variation in age and body composition, the relation between the body impedence corrected for fat-free mass and body height (the specific body impedance) and the calculated ratio of intra-cellular water to total body water, was found to be similar to that in blood. From these observations a regression model was developed and applied to body compositional data of several groups of subjects before and after weight loss caused by water losses. It was possible to calculate at a group level the losses of intra- and extra-cellular water, which confirms the applicability of the model. It is concluded that the validity of the predicted fat-free mass or total body water from body impedance is largely dependent on the water distribution in the measured subjects. This means that age- and sex-specific prediction formulas have to be used for the assessment of the body composition and that the bio-electrical impedance method is only with caution applicable in subjects with a disturbed water distribution as in oedema, pregnancy and dehydration.