In diabetic autonomic neuropathy, abnormal circadian patterns of blood pressure and sympathovagal balance with reduced fall of blood pressure and prevalence of sympathetic activity during the night have been described. To correlate the abnormalities of blood pressure to those of sympathovagal balance, we simultaneously performed 24-h noninvasive monitoring of blood pressure and ECG in 25 diabetic patients (45.6 ± 13.6 yr of age with a 17.6 ± 9.1 yr duration of diabetes) with various degrees of cardiovascular reflex impairment. Autoregressive power spectrum analysis of RR interval variability was applied to 24-h ECG recordings to obtain for day and night periods the mean power of low- (0.03–0.15 Hz) and high-frequency (0.18–0.40 Hz) components, which are relative markers of sympathetic and vagal activity, respectively, and their ratio (low frequency/high frequency), assumed as index of sympathovagal balance. Diabetic patients showed a lower percentage of day-night change in systolic blood pressure (9 ± 5.48 vs. 11.6 ± 4.78%, P < 0.037), a lower day low frequency (5.9 ± 0.81 vs. 6.62 ± 0.73 In-ms2P < 0.001), a lower night high frequency (6.06 ± 0.71 vs. 6.52 ± 0.85 In-ms2P < 0.05), a lower day low frequency:high frequency ratio (1.82 ± 1.77 vs. 3.05 ± 1.82, P < 0.01), and a lower percentage of day-night change in low-frequency:high frequency ratio (– 13.4 ± 109.9 vs. 28.7 ± 29.7%, P < 0.05), when compared with control subjects. Day-night change in low frequency:high frequency ratio correlated to day-night change in systolic blood pressure (r = 0.52, P < 0.01) and diastolic blood pressure (r = 0.48, P < 0.015). In conclusion, in diabetic patients the degree of loss in day-night rhythm of blood pressure is associated with a proportional nocturnal sympathetic predominance. Decreased blood pressure fall combined with relative sympathetic prevalence during the night might represent a risk factor for cardiovascular accidents and could modify the circadian pattern of cardiovascular events in the diabetic population.