The purpose of this investigation was to determine whether maximal oxygen consumption (VO2max) differed between two selected groups of black and white children and whether a difference existed to determine whether it was related to hematologic profiles, body composition, and/or physical activity/inactivity level. Forty-five prepubertal and 42 pubertal, clinically normal black and white children participated. Dual-energy x-ray absorptiometry was used to determine body composition. A computed tomography scan of the abdomen was used to determine visceral adipose tissue and s.c. adipose tissue. Daily physical activity/inactivity was assessed by questionnaire. Black prepubertal and pubertal children had lower VO2max values when compared with white children (28.8 +/- 7.8 versus 35.0 +/- 6.5 mL . kg(-1) . min(-1), p < 0.01; 33.7 +/- 6.4 versus 40.4 +/- 10.2 mL . kg(-1) . min(-1), p < 0.05; respectively). Black prepubertal and pubertal children had lower Hb concentrations ([Hb]) and hematocrits than white children (prepubertal: 12.1 +/- 0.5 versus 12.8 +/- 0.9 g/dL, p < 0.001; 35.6 +/- 1.4 versus 37.4 +/- 2.3%, p < 0.01, respectively; pubertal: 13.0 +/- 0.9 versus 13.6 +/- 0.7 g/dL, p < 0.05; 37.7 +/- 2.5 versus 39.5 +/- 2.1%, p < 0.05, respectively). In conclusion, these findings indicate that black prepubertal and pubertal children had lower VO2max when compared with their white peers matched for age, pubertal stage, and body mass index. This difference in VO2max could be attributed at least in part to comparatively lower [Hb] and more sedentary lifestyle in the black children. Further investigations should study Hb flow rate (a function of [Hb] x maximal cardiac output) in black and white children as it relates to VO2max.