Background/objectives Survival rates in children diagnosed with malignant brain tumors exceed 70%. A higher risk of dyslipidemia, central obesity, and insulin resistance has been reported among these children. We investigated substrate utilization during submaximal exercise. Design/methods Ten brain tumor survivors and 10 healthy children were matched by sex, age, and Tanner stage. Participants completed a submaximal incremental exercise test to determine their fat and carbohydrate oxidation rates. Results The relative oxygen volume (VO2) peak was significantly higher in the control group than in the survivors of childhood brain tumors (43.3 ± 11.9 and 32.4 ± 10.2 mL/kg /min, P = .04). At the same relative exercise intensity, there was no difference in the carbohydrate or lipid oxidation rate between the two groups, or in the maximal fat oxidation (MFO) rate, or in the heart rate or percentage of VO2 peak to reach MFO. Healthy children achieved MFO at significantly higher muscular power than did brain tumor survivors (47.9 ± 20.8 and 21.8 ± 9.6 W, P = .003). Conclusion Because child brain tumor survivors are less physically fit than healthy children, and substrate utilization during submaximal exercise is not different, physical activity should be promoted for child brain tumor survivors.