Post-exercise leucocytosis has been found in children and adults in response to exercise. When normalized to the work performed, children demonstrate a lower lactate response to exercise than in adults. This study examines the association between leukocyte and lactate response in children following exercise. 148 healthy children participated in this study. Each subject performed a ramp exercise on a cycle ergometer followed by ten 2-minute bouts of constant work rate separated by 1-minute rest intervals. Lactate, leukocyte and leukocyte subtype levels were taken pre- and post-exercise. Older children showed a significantly higher pre-to post-exercise leukocyte compared to younger children (3599 ± 165 cells/mL, 2544 ± 163, p<0.0001). Compared to older children, younger children demonstrated a smaller median fold change in lactate (2.7 (1.4-8.1), 4.1 (1.7-29.6)) and leukocyte levels (1.4 (1.0-2.1), 1.6 (1.2-2.8)) after exercise, with a larger leukocyte to lactate fold change ratio.