Ambulatory blood pressure (AMBP) is of particular interest in diabetes because of the close association between elevated BP and diabetic nephropathy and the attenuated night drop in some diabetic subgroups: (1) Normoalbuminuric patients: If standardized for type of day (work or day off), coefficient of variation (CV) for 24 h AMBP is 2%-3% and 5%-6% for night/day ratio. The male-female difference in AMBP seen in healthy subjects is reduced in diabetes. Smoking did not significantly affect AMBP. AMBP is increased in patents with high normal urinary albumin excretion (UAE). Night/day ratio of AMBP and night heart rate is higher in long than short term diabetic patients. This difference in night/day ratio is not significant if the slightly higher UAE in long-term patients is accounted for. (2) Microalbuminuric patients: Diastolic night/day ratio is increased compared with healthy controls, with the value for normoalbuminuric patients in between. A large overlap between groups is evident. Thus the prognostic value of a single abnormal night/day ratio is doubtful. If divided into dippers and nondippers, no difference in extracellular- or plasma volume is found, but nondippers have a lower plasma aldosterone and arginine vasopressin level, possibly to counteract volume expansion. (3) Patients with overt nephropathy: A marked increased in AMBP and a clear reduction of the nocturnal blood pressure fall is seen. In conclusion, AMBP (but not night/day ratio) is highly reproducible. The association between elevated AMBP, elevated night/day ratio, and pathological UAE is detectable even in normoalbuminuric patients. The prognostic importance of abnormal circadian variation of BP is unsettled.