Purpose: The main objective of this study was to determine the reliability of measuring treadmill exercise economy (V̇O(2submax)) and the maximal oxygen uptake (V̇O(2max)) in prepubertal girls tested twice, 6 wk apart. We also wanted to examine the percentage of young girls who were able to reach the criteria for achieving V̇O(2max) and to describe methods that would allow a high proportion of young children to achieve criteria for reaching a true V̇O(2max). Methods: We studied 61 normal-weight, prepubertal girls with a mean (± SD) age 7.3 ± 1.3 yr (range 4.8 to 10.3 yr). V̇O(2submax) was determined while walking for 4 min at 2.5 mph with 0% grade. V̇O(2max) was measured during a progressive, all-out, continuous treadmill test using standardized procedures and criteria. Heart rate (HR) was measured using a Polar monitor. Respiratory rate (RR), respiratory exchange ratio (RER), ventilation (V), and V̇O2 were measured using a Sensormedics metabolic monitor. Results: There were no significant differences between visits 1 and 2 in mean HR, RR, RER, V, V̇O(2submax), (421 vs 422 mL·min-1, respectively), and V̇O(2max) (1036 vs 1049 mL·min-1, respectively). Intra-individual coefficients of variation (CV) between visits 1 and 2 for submaximal tests were: HR = 5.1%, RR = 12.4%, RER = 7.2%, V = 12.5%, and V̇O2 = 12.4%. Intra-individual CVs for the maximum tests were: HR(max) = 2.1%, RR(max) = 10.8%. RER(max) = 5.3%, V(max) = 11.7%, and V̇O(2max) = 7.5%. A high proportion of the girls reached criteria for V̇O(2max) [R > 1.00, HR > 85% of age predicted maximum, and plateauing of V̇O(2max)] in both visits: 99% reached one of three criteria, 92% reached two of three criteria, and 70% reached all three criteria. Twenty girls [mean-age 7.2 ± 1.2 yr] reached at least two criteria in both visits, whereas 32 girls [mean (± SD) age 8.6 ± 1.0 yr] reached three criteria in both visits. Conclusion: Exercise measurements using treadmill testing were reliable in healthy, normal-weight, prepubertal girls. Older girls when compared to the younger girls were able to reach criteria for V̇O(2max) more often. Thus, we recommend that one testing should give researchers an accurate measure of walking economy and aerobic capacity, and that two criteria are enough for determining V̇O(2max).