OBJECTIVE: To evaluate autonomic nervous function in simple diabetes mellitus patients, diabetic nephropathy(DN) patients and diabetes mellitus patients with accompanying coronary heart disease(DC) by studying heart rate variability (HRV) and cardiovascular reflex tests. METHODS: 57 type II diabetes mellitus patients were divided into 3 groups: 29 of simple diabetes mellitus, 17 of DC, and 11 of DN. The three groups were matched to 15 healthy controls for age, HRV and three cardiovascular reflex tests were performed in all the patients. RESULTS: The overall frequency of abnormal autonomic nervous function in 57 diabetes mellitus patients was 45.6%. All the diabetes mellitus groups especially the DN group were lower than a normal control group in time domain indexes and nonlinear dynamics indexes. As the patterns of R-R interval Poincare plots were concerned, the control group was mainly of comet-shaped pattern, the simple diabetes mellitus group was mainly of torpedo-shaped pattern, while the DN group and the DC group had more irregular patterns. CONCLUSION: Decrease in HRV is an early clinical marker for evolving diabetic autonomic neuropathy. This method may be helpful for evaluating organ dysfunction in diabetes mellitus patients.