Comparisons were effected of the following four methods of estimating the percent body fat (%BF) of 12 highly trained male endurance athletes (mean +/- SD = 2.20 +/- 4.9 years, 176.8 +/- 5.9 cm 64.2 +/- 6.4 kg): underwater weighing (UWW), total body water (TBW), total body potassium (TBK) and dual-energy X-ray absorptiometry (DEXA). The DEXA mean of 6.8% BF was significantly less (P < 0.05) than those estimated via UWW: 9.7% BF; TBW: 10.6% BF (fat-free mass of FFM = 72.0% H2O); and TBK: 9.7% BF (FFM = 66.6 mmol K.kg-1). Nevertheless, the DEXA % BF correlated 0.746 and 0.737 (both P < 0.01) with those from UWW and TBW, respectively; these were the only correlation coefficients to attain statistical significance (P < or = 0.05). Despite the similar means for UWW, TBW and TBK, 12 of the 36 individual differences between these three methods ranged from 3.2 to 10.4% BF. A critical assumption of UWW, which is regarded by many as the criterion method for the estimation of % BF, is that the FFM has a density of 1.100 g.cm-3. Use of in vivo-measured TBW and bone mineral (via DEXA) for the computation of FFM densities for our subjects, while assuming that the two other components of the FFM (protein and non-bone mineral) remained constant, resulted in scores ranging from 1.09541 to 1.10246 g.cm-3 (mean +/- SD = 1.09881 +/- 0.00254 g.cm-3). FFM and % BF differences between use of a constant FFM density of 1.100 g.cm-3 and the individual values ranged from -1.02 to 0.57 kg (mean +/- SD = -0.28 +/- 0.60 kg) and from -0.9 to 1.7% BF (mean +/- SD = 0.5 +/- 0.9% BF), respectively. It may be concluded that with young male athletes: (1) use of constants based on normal male cadavers yielded similar group means for % BF determined by UWW, TBW and TBK but the DEXA % BF correlated significantly with those from UWW and TBW; and (2) in vivo measurements of individual differences in TBW and bone mineral support the use of conventional UWW for the estimation of % BF.