We used bioelectrical impedance analysis (BIA) as an exploratory tool to monitor the changes in body composition induced by a short-term (3-wk) weight reduction (energy-restricted diet, moderate aerobic exercise conditioning and psychological counselling) in 175 highly obese subjects (body mass index, BMI=41.7+/-5.8 kg/m2). The decrease in weight and BMI after the weight reduction program was 3.4% (geometric mean, p<0.0001) and 3.7+/-1.3 kg/m2 (mean+/-SD, p<0.0001), respectively. Bioelectrical impedance (Z) increased of about the same value at each of the measured frequencies (from 6+/-10% at 5 kHz to 5+/-9% at 100 kHz, mean+/-SD, p<0.0001). A statistically significant increase in Z5:Z100 was also seen (p<0.0001), but its clinical significance is questionable owing to its low absolute value (<1%). Taken together, these data suggest that no clinically relevant change in body water distribution occurred in our subjects as a result of the weight reduction program. However, the changes in Z did not satisfactorily predict the changes in anthropometric dimensions despite the evidence of a substantial association between Z and anthropometry both before and after the weight reduction program. Thus, accurate predictions of body composition changes in obese subjects may require more than two BIA measurements so as to have a better description of the weight-losing process.