In healthy individuals compared to normoalbuminuric patients, day time blood pressure (BP) is indistinguishable between the groups in most studies, while a slightly higher nocturnal systolic BP is a common finding. Despite a comparable clinic BP both day and night BP are higher in microalbuminuric patients, and the diastolic night/day ratio is clearly elevated as compared with healthy controls. In normo- and microalbuminuric patients ambulatory blood pressure (AMBP) correlates with urinary albumin excretion (UAE). The evolution of AMBP in a three year follow-up study presented evidence that in healthy individuals the increase in 24 hour systolic BP was approximately 1 mm Hg per year and was almost identical to the increase in persistent normoalbuminuric patients. In contrast, patients who progressed to microalbuminuria exhibited a fourfold higher increase in 24 hour AMBP compared to the persistent normoalbuminuric patients. Ordinary clinic measurements did not reveal this difference. There was no difference in initial 24 hour AMBP in progressors and non-progressors. Our study favors the concept of a simultaneous rise in BP and UAE. In overt renal disease, AMBP is significantly higher than in microalbuminuric patients. The circadian variation of blood pressure is blunted particularly in patients with advanced diabetic nephropathy and antihypertensive treatment. AMBP in type 1 diabetic patients allows: (1) the detection of minor but clinical relevant elevations of BP; (2) to disclose and association between BP and UAE in cross sectional studies; (3) to give a description of the blunted diurnal BP rhythm in patients with incipient or overt diabetic nephropathy.(ABSTRACT TRUNCATED AT 250 WORDS)