Abstract The purpose of this investigation was to determine whether maximal oxygen consumption (VO2max) differed between clinically obese black and white children and if a difference existed to determine whether it was related to hematological profiles and/or physical activity/inactivity levels. Twenty-three black and 21 white adolescents were matched for age, BMI, and Tanner stage (II-V). Body composition was determined by DEXA and CT scan. Daily physical activity/inactivity was assessed by questionnaire. VO2max was assessed using the Bruce treadmill protocol. Black participants had significantly lower VO2max and VO2maxFFM values when compared with white adolescents (26.1 ± 4.2 versus 29.9 ± 3.1 mL • kg−1 • min−1; 48.3 ± 8.8 versus 55.6 ± 5.2 mL • kgFFM−1 • min−1, respectively). Black adolescents also had significantly lower Hb concentrations ([Hb]) than white children (12.7 ± 1.3 versus 13.4 ± 0.7 g/dL). Black adolescents were more physically inactive than their white peers. VO2max correlated with [Hb] for the combined groups. Obese black adolescents had lower VO2max compared with white children and this difference was explained, in part, by the lower [Hb] observed in the black participants. Further investigations should study Hb flow rate (a function of [Hb] • maximal cardiac output) and physical activity/inactivity patterns in obese black and white children as it relates to VO2max.