Existing equations for bioelectrical impedance analysis (BIA) are of limited use when subjects age or become overweight because these equations were developed in young, normal-weight subjects and are not valid in elderly or overweight people. The purpose of this study was to validate a single BIA equation in healthy white subjects aged 22–94 y with a body mass index between 17.0 and 33.8 kg/m². Healthy subjects (202 men and 141 women) aged 20–94 y were measured by two methods: fat-free mass (FFM) was measured by dual-energy x-ray absorptiometry (Hologic QDR-4500) and by a bioelectrical impedance analyzer (Xitron 4000B). Validity of BIA was assessed by double cross validation. Because correlations were high (r = 0.986–0.987) and prediction errors low, a single equation was developed using all subjects, as follows: FFM = −4.104 + (0.518 × height²/resistance) + (0.231 × weight) + (0.130 × reactance) + (4.229 × sex: men = 1, women = 0). FFM predicted with dual-energy x-ray absorptiometry was 54.0 ± 10.7 kg. BIA-predicted FFM was 54.0 ± 10.5 kg (r = 0.986, standard error of the estimate = 1.72 kg, technical error = 1.74 kg). In conclusion, the new Geneva BIA equation was valid for prediction of FFM in healthy adults aged 22–94 y with body mass indexes between 17.0 and 33.8 kg/m². Inclusion of reactance in the single prediction equation appeared to be essential for use of BIA equations in populations with large variations in age or body mass.