The aim of the present study was to evaluate the influence of autonomic nervous system dysfunction on work capacity in children and adolescents with Type 1 (insulin-dependent) diabetes. Fifteen patients with autonomic dysfunction (abnormal autonomic tests, age: 14.9 +/- 2.3 years), 35 patients without autonomic dysfunction (normal autonomic tests, age: 15.2 +/- 2.5 years), and 25 non-diabetic subjects (age:15.0 +/- 2.3 years) were investigated. Resting heart rate, deep breathing heart rate variation, standing/lying heart rate ratio, decrease in blood pressure during orthostasis, and increase in blood pressure during sustained handgrip were used to assess cardiovascular autonomic dysfunction. Physical work capacity at heart rate of 170 min-1 was determined by bicycle ergometry. Glycated haemoglobin level was higher in patients with than without autonomic dysfunction (12.3 +/- 3.1 vs 9.4 +/- 2.9%, p = 0.04). Patients with autonomic dysfunction had significantly lower physical work capacity at heart rate of 170 min-1 than those with normal autonomic function or non-diabetic subjects (0.81 +/- 0.12 vs 1.49 +/- 0.16 and 1.54 +/- 0.20 W kg-1 p = 0.01). Physical work capacity at heart rate of 170 min-1 was related to glycated haemoglobin level (r = -0.55, p = 0.01), to resting heart rate (r = 0.57, p = 0.01), and to deep breathing heart rate variation (r = 0.51, p = 0.02). In conclusion, impaired work capacity is associated with poor blood glucose control and cardiovascular autonomic dysfunction. Autonomic tests can help to identify those patients who may need special consideration during exercise.