1. Autonomic abnormalities are frequent in adult patients with diabetes mellitus and progress slowly; little is known about frequency and progression of autonomic abnormalities in childhood. 2. To assess whether autonomic abnormalities are already present in childhood, we evaluated the cardiovascular reflexes, the spectral analysis of spontaneous fluctuations in RR interval and blood pressure (low- and high-frequency), and the baroreflex sensitivity at rest, and after vagal (controlled breathing) and sympathetic activation (tilting) in 25 adolescents with Type I diabetes mellitus, aged 10-17 years, at baseline and after 18 months follow-up, and in 20 age- and sex-matched controls. 3. Cardiovascular reflexes were similar in both patients and controls. Similar significant changes in percentage low- and high-frequency (P < 0.005) from rest to tilting and to control breathing were observed in both patients and controls. The baroreflex sensitivity was also similar in patients and controls. Mild and non-systematic correlations were observed between autonomic tests and disease duration or metabolic control. After 18 months follow-up no changes were observed in any of the measured variables. Correlations with metabolic control remained unchanged. 4. These results indicate a substantial stability of cardiovascular autonomic function in childhood diabetes, and suggest that autonomic abnormalities are likely to develop at an older age.