BACKGROUND: With growing age, values for oxygen uptake decrease for the same level of exercise. However, some children with normal heart and normal maximal oxygen uptake complain of exertional fatigue. AIM: To evaluate the energy expenditure during submaximal treadmill exercise. SUBJECTS AND METHODS: In 20 children with exertional fatigue (mean age at testing: 7.9 +/- 1.8 years), oxygen uptake and respiratory gas exchange were assessed breath by breath. A graded exercise test was performed until exhaustion. Patients were compared to an age- and gender-matched control group (8.4 +/- 1.9 years, p = 0.45). RESULTS: Maximal oxygen uptake in patients (48.6 +/- 7.1 mL O(2)/min/kg) was similar to normal controls (47.4 +/- 5.2 mL/min/kg, p > 0.25). The inclination of the treadmill at maximal exercise was significantly (p = 0.02) lower in patients (12.6 +/- 4.9%) versus controls (16.0 +/- 3.5%). During submaximal exercise, oxygen uptake (expressed as mL/min/kg or as a percent of maximal oxygen uptake) was significantly higher (p < 0.001) in patients compared to normal controls. CONCLUSION: Children with exertional fatigue as compared to healthy controls, have a higher oxygen uptake for the same level of exercise and therefore perform at each level of exercise closer to their maximal exercise capacity. This may in part explain subjective complaints of poor exercise tolerance.