Immune responses have been examined in 11 children aged 10.3 ± 0.6 years before and after 12 weeks of aerobic training. Initial resting data showed high total lymphocyte, CD3+ and CD8+ counts, a low CD4+/CD8+ ratio and a low CD25+ count relative to young adults. Acute exercise (30 min at ventilatory threshold) initially increased CD4+, CD8+, and CD56+ counts, and decreased CD4+/CD8+ ratio, but CD56+ count did not decrease during recovery. After training, relative aerobic power remained unchanged at 50 ±3 ml · kg−1 · min−1. However, resting leukocyte, CD3’ and CD25’ counts were decreased, and acute exercise induced smaller changes in leukocyte and subset counts. We conclude that immune responses to exercise are generally similar in children and young adults.