Estimates of body composition by densitometry were made in 84 apparently healthy subjects (42 men, 42 women) with a mean age of 40 +/- 6 years (mean +/- s.d.), before and after weight loss. The initial body mass index (BMI) was 30.7 +/- 2.3 kg/m2 and the achieved weight loss on a 4.2 MJ/day energy deficit diet for 13 weeks was 12.2 +/- 3.7 kg. The results by densitometry were compared with estimates obtained by four other techniques: deuterium oxide dilution, skinfold thickness, bioelectrical impedance (three equations) and BMI (two equations). The fat-free mass (FFM) loss estimated by densitometry in men and women was 2.8 +/- 1.8 kg and 1.3 +/- 1.3 kg respectively. The dilution technique gave comparable results with densitometry. The losses of FFM assessed by skinfold thicknesses, BMI and impedance equations were almost similar, but significantly larger than the reduction in FFM measured by densitometry. These deviations were mainly the result of significantly larger differences from densitometry before compared to after weight loss. No correlation was found between change in FFM by densitometry and change in resistance measured by the bioelectrical impedance method in both sexes. It is concluded that application of published prediction formulae in weight loss studies are less appropriate and will lead to changes in FFM that are significantly different from the changes estimated by densitometry or deuterium oxide dilution.