BACKGROUND: Changes of body composition occur with aging and influence health status. Thus accurate methods for measuring fat-free mass (FFM) in the elderly are essential. OBJECTIVE: The purpose of this study was to compare FFM obtained by three bioelectrical impedance analysis (BIA) published formulas specific for the elderly and one equation intended for all age groups, with FFM derived from dual-energy X-ray absorptiometry (FFM(DXA)), in healthy elderly subjects. METHODS: Healthy Caucasian subjects over 65 years (106 women, age 75 +/- 6.2, body mass index 25.2 +/- 4.1 and 100 men, age 74.6 +/- 6.6, body mass index 25.8 +/- 3.0) were measured by DXA (Hologic QDR-4500) and BIA (Xitron, 50 kHz). FFM(BIA) was calculated by the published formulas of Deurenberg, Baumgartner, Roubenoff and Kyle and compared to FFM(DXA) by a Bland-Altman analysis. RESULTS: The Deurenberg and Roubenoff BIA formulas underestimated FFM compared to DXA by -7.1 and -2.9 kg in women and -6.7 and -2.3 kg in men, respectively. The Baumgartner formula overestimated FFM by 4.3 kg in women and 1.4 kg in men. The Kyle formula showed differences of 0.0 kg in women and 0.2 kg in men, and the limits of agreement of FFM(BIA (Kyle)) relative to FFM(DXA) were -3.3 and +3.3 kg for women and -3.8 and +4.3 kg for men. CONCLUSION: The Kyle BIA formula accurately predicts FFM in elderly Swiss subjects between 65 and 94 years, with a body mass index of 17 to 34.9 kg/m(2). The other BIA formulas developed especially for the elderly are not valid in this population.