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To compare single estimations of fat-free mass (FFM) and to track FFM using single-frequency bioelectrical impedance analysis (BIA) and dual-energy X-ray absorptiometry (DXA) compared with a four-compartment (4C) model in healthy elderly Americans.Thirty-four men and thirty-eight women (Caucasian, X65 years) were included in the study. Subjects participated in either the control group or the exercise group. All testing and training took place during the 21-week investigation. Body composition assessments using nine BIA equations, DXA and a 4C model were performed during weeks 1, 12 and 24 of the study.Single estimations for DXA and BIA produced high r values (0.79-0.95) and low standard error of estimate values (1.62-3.3 kg), producing subjective ratings of ‘ideal’ for men and ‘excellent’ for women. Both DXA and two BIA equations revealed the same significance when comparing groups and times with the 4C model. Individual accuracy for tracking changes was similar among BIA equations and DXA compared with the 4C model, with a total agreement of 25% for BIA and 27% for DXA compared with the 4C model.The current data in combination with the reliability errors for both BIA and DXA FFM estimations suggest that individual results should be interpreted with caution if FFM changes are <5kg. However, DXA and BIA are both valid methods that can be used interchangeably to estimate FFM at a single time point or for tracking changes in FFM in small groups (15-22) of healthy American older adults.