The authors used 3-component (3C) Db-mineral-model (Lohman, 1986) reference measures to cross-validate Siri's (1961) 2-component (2C) conversion formula and dual-energy X-ray absorptiometry (DXA) estimates of relative body fat (%BF) for physically active adults. Participants varied in age (18 to 59 y), body fatness, ethnicity (black, Hispanic, white), and physical activity level. The 3C Db-mineral model was used to obtain reference measures of % BF (%BF3C) for comparison with body-composition measures from DXA and hydrodensitometry. For men (n = 110) and women (n= 110), %BF3C (14.0 % BF and 24.4 % BF, respectively) was more accurately estimated by Siri's 2C formula (% FSiri; men, r = 0.97, SEE = 1.77 % BF; women, r = 0.98, SEE = 1.56 % BF) than by DXA (% FDXA; men, r = 0.86, SEE = 3.54 % BF; women, r =0.88, SEE = 3.73 % BF). The average %BFSiri (men, 15.8 % BF; women, 24.7 % BF) and % FDXA (men, 16.2 % BF; women, 26.0 %BF) differed significantly (P < 0.001) from %BF3C. Siri's 2C model estimated the average % BF3C in this sample more accurately than DXA did.