We performed bio-electrical impedance analysis (BIA) on 38 healthy adults with an initial body mass index (+/- s.d.) of 30 kg/m2 (+/- 5) before and after a low calorie diet. Five weeks later a mean weight loss of 4.2 +/- 2.3 kg was observed. According to BIA, fat-free mass (FFM) decreased in 28 subjects and increased in ten. In four cases the reduction was greater than the weight loss. At the end of the ten week diet regimen all 27 subjects followed up demonstrated a further weight reduction. According to BIA, FFM decreased in 22 subjects, increased in four and did not change in one case. In one case the reduction was greater than the weight loss. Although mean FFM values assessed by skinfolds and BIA were not significantly different before and after the weight loss period, no correlations were found among the individual changes in FFM evaluated by the two methods. Our results suggest that single frequency (50 KHz) impedance must be used with care in clinical conditions.