The objective of this study was to evaluate exercise capacity in children and adolescents diagnosed with Chronic Fatigue Syndrome (CFS). We examined 20 patients (12 girls and 8 boys; mean age 14.9 +/- 3.7 years) diagnosed with CFS. Exercise capacity was measured using a maximal exercise test on a bicycle ergometer and an expired gas analysis system. Fatigue was assessed using a questionnaire and a daily activity diary was used to describe activities for three days. Z-scores were calculated using age- and sex-matched reference values. Z-scores in children and adolescents with CFS were - 0.33 +/- 1.0 (p = 0.17) for peak oxygen uptake, - 1.13 +/- 1.41 (p = 0.002) for relative peak oxygen uptake [ml/kg/min] and - 0.93 +/- 1.29 (p = 0.07) for maximal work load. Both heart rate and blood pressure at peak performance were significantly reduced compared to reference values. Fatigue levels were significantly positively associated with age and negatively with blood pressure at peak exercise (p < 0.05). In conclusion maximum exercise testing was feasible in young people with CFS. Maximal exercise capacity was only reduced in a minority of the patients and was related to current physical activity levels.