OBJECTIVE To investigate the ability of children to improve aerobic fitness, as indicated by changes in maximaal oxygen uptake (VO2max), after a 12-week period of endurance training. METHODS Longitudinal prospective training with control period. METHODS Middle school physical education class. METHODS Twenty-four girls and 13 boys, ages 10.9 to 12.8 years. METHODS Three 30-minute sessions of aerobic activity weekly for 12 weeks at an intensity-producing a mean heart rate of 166 beats per minute. Maximal treadmill testing was performed 12 weeks before the training program and again at the start and end because each child served as his/her own control. RESULTS Mean VO2max did not change significantly during the control period but rose from 44.7 (5.8) to 47.6 (6.4) mL kg-1min-1 (6.5%) with training (P < .05). No differences in training response were observed relative to sex, pretraining VO2max, or sports participation. CONCLUSIONS These findings support the concept that VO2max can be improved with endurance training during the childhood years, but the degree of aerobic train-ability is limited in healthy, active children.