Objective:To elucidate distinctive patterns of circadian variation of blood pressure in diabetic patients.Design:Circadian variations and indices of blood pressure variability were examined by non-invasive ambulatory blood pressure monitoring. The indices were assessed for a correlation with various possible signs of autonomic neuropathy, including the coefficient of variation of the RR interval and orthostatic hypotension.Methods:The study was conducted on 38 normotensive diabetics and 15 age- and sex-matched non-diabetic controls, all of whom were outpatients at the Third Department of Internal Medicine in Nagoya University Hospital. Ambulatory blood pressure measurements were taken in all subjects and the coefficients of variation in the RR interval and postural changes in blood pressure were determined.Results:The diabetic patients demonstrated a different pattern of circadian variation in blood pressure from that of the non-diabetics. Diurnal-nocturnal differences were significantly smaller in diabetics than in non-diabetic controls. Statistically significant correlations were found between diurnal-nocturnal differences in blood pressure and the duration of diabetes, glycaemic control, the coefficient of variation in the RR interval and the change in blood pressure on standing. A subgroup of diabetic patients with severe autonomic neuropathy showed the lowest diurnal—nocturnal differences.Conclusions:In diabetic patients with autonomic neuropathy circadian variations in blood pressure are blunted. Ambulatory blood pressure monitoring can detect these abnormal variations and therefore may be useful in evaluating diabetic autonomic neuropathy.