The aim was to assess cardiovascular autonomic dysfunction in children and adolescents with diabetes mellitus. A total of 110 children and adolescents with type 1 (insulin dependent) diabetes (aged 6 to 18 years) and 130 non-diabetic controls were studied. Resting heart rate, heart rate variation to deep breathing, heart rate response to standing from a lying position, fall in systolic blood pressure on standing, and rise in diastolic blood pressure during sustained handgrip were measured. A reference range of results was obtained in the controls. Diabetic children had significantly increased resting heart rate [92.4 (SEM 2.5) v 84.2 (2.2) beats/min], decreased deep breathing heart rate variation [25.3 (0.9) v 32.8 (0.6) beats/min], and lower standing/lying heart rate ratio [1.23 (0.04) v 1.31 (0.03)] compared with controls. 46 diabetic children (42%) had at least one abnormal autonomic test result. Of these, 20 (15%) had only one abnormal test and 26 (24%) had two or more abnormal tests. Using multiple logistic regression analysis, longer diabetes duration and worse long term metabolic control were independently predictive of cardiovascular autonomic dysfunction as the dependent variable [adjusted OR (95% CI): 2.9 (1.1-5.9) and 3.3 (1.2-6.4), respectively]. Cardiovascular autonomic dysfunction is not rare in children with diabetes. Efforts should be made to maintain the best metabolic control to prevent or delay these complications.