The purpose of the present study was to test the validity of the electrical impedance method for estimation of total body water and lean body mass in adult Danes with large differences of obesity and fat distribution, and to develop algorithms for estimation of body water, lean body mass and fat from measurement of impedance. The results of the electrical impedance method were compared in 139 Danes aged 35-65 years, to those of a four-compartment-model based on measurements of both total body potassium (whole body counting) and total body water (dilutometry). The comparison confirmed the validity of the impedance method. Equations for predicting body water and lean body mass in Danes are given. A significant difference was found between two instruments of the same model. In spite of the fact that the test resistance supplied by the manufacturer gave identical measurements, measurements on humans diverged on average by 31 ohm. A cross-validation study showed that most of the algorithms found in the literature for predicting lean body mass from impedance yield reliable results.