The current study sought to examine in a large series of diabetic patients the prevalence of symptoms of autonomic neuropathy and subclinical cardiac autonomic neuropathy (CAN) and their determinants, particularly the influence of diabetes duration, obesity, and microangiopathic complications. Three hundred ninety-six patients, 245 type 1 and 151 type 2, were recruited in 7 French departments of diabetology. CAN was detected by measuring heart rate variability during 3 standardized tests: deep-breathing, Valsalva, and lying-to-standing tests. At least 24.5% of the patients had one or more symptoms suggesting overt autonomic neuropathy. They were older than those free of dysautonomic symptom (P<.001). The deep-breathing test correlated negatively with body mass index (BMI) in type 2 diabetic patients (P<.0001). In the whole population, the deep-breathing and Valsalva tests correlated negatively with diabetes duration (P=.0004 and.019, respectively) and the log urinary albumin/creatinine ratio (P<.002 and.001, respectively). The prevalence of CAN (51%) was higher than the prevalence of other diabetic complications. The rate of moderate and severe CAN (defined by 2 or 3 abnormal CAN function tests) was higher in type 1 than in type 2 diabetic patients (P=.031). It correlated with diabetes duration (P=.026) and was higher in the patients with retinopathy than in those without (P=.035). Among type 2 diabetic patients, the prevalence of CAN was higher in the obese ones (P=.033); in a logistic regression taking age, diabetes duration, and obesity as independent variables, CAN was associated independently with obesity (P=.034). Mild or moderate CAN was found in 33.8% and 13.0% of the 80 patients with diabetes duration less than 18 months. We conclude that CAN is found early in the course of diabetes and should be considered as a prognostic marker of microangiopathic complications. Obesity could be involved in the impairment of CAN function in type 2 diabetics and body weight control could provide an approach to reducing neuropathic complications.