Abstract-To focus on early cardiac and vascular autonomic dysfunction that might complicate type 1 diabetes mellitus in children, we planned an observational, cross-sectional study in a population of 93 young patients, under insulin treatment, subdivided in 2 age subgroups (children: 11.5Ϯ0.4 years; adolescents: 19.3Ϯ0.2 years). Time and frequency domain analysis of RR interval and systolic arterial pressure variability provided quantitative indices of the sympatho-vagal balance regulating the heart period, of the gain of cardiac baroreflex, and of the sympathetic vasomotor control. Sixty-eight children of comparable age served as a reference group. At rest, systolic arterial pressure and the power of its low-frequency component were greater in patients than in controls, particularly in children (14.0Ϯ2.3 versus 3.1Ϯ0.3 mm Hg 2 ). Moreover, baroreflex gain was significantly reduced in both subgroups of patients. Standing induced similar changes in the autonomic profiles of controls and patients. A repeat study after 1 year showed a progression in low-frequency oscillations of arterial pressure and a shift toward low frequency in RR variability. Data in young patients with type 1 diabetes mellitus show a significant increase in arterial pressure, a reduced gain of the baroreflex regulation of the heart period, and an increase of the low-frequency component of systolic arterial pressure variability, suggestive of simultaneous impairment of vagal cardiac control and increases of sympathetic vasomotor regulation. A repeat study after 1 year shows a further increase of sympathetic cardiac and vascular modulation, suggesting early progression of the autonomic dysfunction. Key Words: autonomic nervous system Ⅲ baroreflex Ⅲ children Ⅲ diabetes mellitus Ⅲ sympathetic nervous system Ⅲ vasculature C ardiovascular autonomic neuropathy is a severe, frequently underrecognized complication of diabetes mellitus, of which the presence approximately doubles the all-cause mortality risk. 1 Traditionally, clinical markers of diabetic cardiovascular autonomic neuropathy are based on a battery of tests 2 requiring the active collaboration of patients; however, new methods have been described to assess the integrity of autonomic function, such as spectral analysis of RR interval variability 3,4 or baroreflex sensitivity (BRS) 5,6 , that can be obtained automatically from computer analysis of noninvasive ECG and arterial pressure wave recordings, without needing the active participation of patients. These latter methods have been considered more sensitive 7 than traditional tests and, thus, could be particularly suited to assess initial changes in autonomic performance, particularly in young patients affected by type 1 diabetes mellitus (T1DM).In adults, available data suggest that, with time, cardiovascular autonomic neuropathy 8 becomes characterized by reduced vagal control of the sinoatrial node, as expressed by reduced RR variance and baroreflex gain, together with impaired vascular regulation, as exemplified by reduced orthostati...