Seventeen type I male diabetic adolescents and 17 control subjects matched for age, height, and weight were submitted to maximal exercise on a bicycle ergometer. The diabetic subjects were divided into two groups according to their degree of metabolic control using total glycosylated hemoglobin (HbA1): group 1, diabetics with HbA1 less than 8.5% (n = 9) and group 2, diabetics with HbA1 greater than 8.5% (n = 8). Oxygen uptake, pulmonary ventilation, and heart rate were recorded at rest and at maximal load. Glucose, lactate, and free fatty acids were determined in blood before and after exercise. Maximal work load and oxygen uptake were significantly lower in the two diabetic groups than in the healthy controls. An inverse relationship was observed between HbA1 concentration and the maximal work load (r = -0.63; P less than 0.01). It can be concluded that diabetic adolescents should obtain the best possible degree of metabolic control to improve their performances.