The purpose of this study was to compare the accuracy, precision, and bias of fat mass (FM) as assessed by dual-energy X-ray absorptiometry (DXA), hydrostatic weighing (HW), air-displacement plethysmography (PM) using the BOD POD body composition system and total body water (TBW) against the four-compartment (4C) model in 25 children (11.4 +/- 1.4 yr). The regression between FM by the 4C model and by DXA deviated significantly from the line of identity (FM by 4C model = 0.84 x FM by DXA + 0.95 kg; R(2) = 0.95), as did the regression between FM by 4C model and by TBW (FM by 4C model = 0. 85 x FM by TBW - 0.89 kg; R(2) = 0.98). The regression between FM by the 4C model and by HW did not significantly deviate from the line of identity (FM by 4C model = 1.09 x FM by HW + 0.94 kg; R(2) = 0. 95) and neither did the regression between FM by 4C (using density assessed by PM) and by PM (FM by 4C model = 1.03 x FM by PM + 0.88; R(2) = 0.97). DXA, HW, and TBW all showed a bias in the estimate of FM, but there was no bias for PM. In conclusion, PM was the only technique that could accurately, precisely, and without bias estimate FM in 9- to 14-yr-old children.