Changes in body weight in relation to changes in body impedance were studied in six male and four female patients before and during dialysis. Before dialysis, fat-free mass from skinfolds was 46.5 +/- 6.8 kg, and total body water calculated from bio-electrical impedance was 36.9 +/- 5.2 kg. Body water expressed as a percentage of fat-free mass was calculated to be 80 +/- 4%. Mean weight loss, which was assumed to be only loss of water, was 2.7 +/- 0.7 kg. Total body impedance increased by 80 +/- 16 omega from 495 +/- 39 omega to 575 +/- 47 omega. Weight loss and increase in body impedance were highly correlated (r = -0.91, P less than 0.001). However, based on prediction formulas for total body water from bio-electrical impedance, the observed weight (water) loss should be associated with an increase in impedance of only 39 +/- 11 omega. It is concluded that the impedance value before dialysis is a relatively low value, due to the low specific resistivity of the excess of extra-cellular water in the body before dialysis. As a consequence, prediction formulas for body composition from bio-electrical impedance, developed in normal hydrated subjects, overestimate body composition (total body water, fat-free mass) in subjects with oedema.