Differences in the mineral fraction of the fat-free mass (M_FFM) and in the density of the FFM (D_FFM) are often inferred from measures of bone mineral content (BMC) or bone mineral density (BMD). We studied the relation of BMC and BMD to the M_FFM and D_FFM in a heterogeneous sample of 216 young men (n = 115) and women (n = 101), which included whites (n = 155) and blacks (n = 61) and collegiate athletes ( n = 132) and nonathletes (n = 84). Whole body BMC and BMD were determined by dual-energy X-ray absorptiometry (DXA; Hologic QDR-1000W, enhanced whole body analysis software, version 5.71). FFM was estimated using a four-component model from measures of body density by hydrostatic weighing, body water by deuterium dilution, and bone mineral by DXA. There was no significant relation of BMD to M_FFM (r = 0.01) or D_FFM (r = 0.06) or of BMC to M_FFM (r = 0.11) and a significant, weak negative relation of BMC to D_FFM (r = 0.14, P = 0.04) in all subjects. Significant low to moderate relationships of BMD or BMC to M_FFM or D_FFM were found within some gender-race-athletic status subgroups or when the effects of gender, race, and athletic status were held constant using multiple regression, but BMD and BMC explained only 10-17% of the variance in M_FFM and 0-2% of the variance in D_FFM in addition to that explained by the demographic variables. We conclude that there is not a significant positive relation of BMD and BMC to M_FFM or D_FFM in young adults and that BMC and BMD should not be used to infer differences in M_FFM or D_FFM.