Dual-energy x-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA) are common methods of body composition assessment, but the agreement between these methods varies. Bias between DXA and single-frequency BIA was evaluated at six different time points in 48 active male and female adults consuming standardized high- and low-carbohydrate diets. It was hypothesized that fixed and proportional biases exist between DXA and BIA, but that the extent of bias does not differ based on sex. Substantial fixed bias was present for estimates obtained by DXA and BIA, and both males and females exhibited proportional bias for fat mass and fat-free mass. The magnitude of bias was greater in females, and only females exhibited proportional bias for body fat percentage. In individuals with less fat mass, the mean difference between DXA and BIA was high, indicating that BIA underestimated fat mass relative to DXA. However, in individuals with greater fat mass, better agreement was seen. Correspondingly, the mean difference in fat-free mass estimates was greater in individuals with less fat-free mass. In some individuals with high quantities of fat mass or fat-free mass relative to the sample, the relationship between devices was reversed such that BIA overestimated fat mass and underestimated fat-free mass. The degree of disagreement between DXA and BIA varies substantially based on body size and sex such that all-encompassing statements regarding the comparability of these technologies cannot presently be made.