The purpose of this study was to examine differences in resting, submaximal, and maximal (VO(2max)) oxygen consumption (VO2) in African- American (n = 44) and Caucasian (n = 31) prepubertal children aged 5-10 yr. Resting VO2 was measured via indirect calorimetry in the fasted state. Submaximal VO2 and VO(2max) were determined during an all out, progressive treadmill exercise test appropriate for children. Dual-energy X-ray absorptiometry was used to determine total fat mass (FM), soft lean tissue mass (LTM), and leg soft LTM. Doubly labeled water was used to determine total energy expenditure (TEE) and activity energy expenditure (AEE). A significant effect of ethnicity (P < 0.01) was found for VO(2max) but not resting or submaximal VO2, with African American children having absolute VO(2max) ~15% lower than Caucasian children (1.21. ± 0.032 vs. 1.43 ± 0.031 l/min, respectively). The lower VO(2max) persisted in African-American children after adjustment for soft LTM (1.23 ± 0.025 vs. 1.39 ± 0.031 l/min; P < 0.01), leg soft LTM (1.20 ± 0.031 vs. 1.43 ± 0.042 l/min; P < 0.01), and soft LTM and FM (1.23 ± 0.025 vs. 1.39 ± 0.031 1/min; P < 0.01). The lower VO(2max) persisted also after adjustment for TEE (1.20 ± 0.02 vs. 1.38 ± 0.0028 l/min P < 0.001) and AEE (1.20 ± 0.024 vs. 1.38 ± 0.028 l/min; P < 0.001). In conclusion, our data indicate that African-American and Caucasian children have similar rates of VO2 at rest and during submaximal exercise, but VO(2max) is ~15% lower in African-American children, independent of soft LTM, FM, leg LTM, TEE, and AEE.