In this article, we analyze the blood pressure (BP) threshold for the start of antihypertensive treatment in insulin-dependent diabetes mellitus (IDDM) patients, with particular emphasis on those with persistent microalbuminuria or proteinuria (incipient and overt nephropathy, respectively). In such individuals, there is a clear increase in the prevalence of hypertension and in actual measured BP values that is not observed in normoalbuminuric patients. In 94 young healthy adults (less than 45 yr of age), average mean +/- SD arterial pressure (MAP; diastolic + 1/3 pulse pressure) was approximately 90.0 +/- 8.1 mmHg, closely corresponding to large population studies. In microalbuminuric IDDM patients, MAP values between approximately 105 and approximately 95 mmHg have been found in different studies, and the level has progressively decreased in various studies between 1984 and 1990 with similar BP-measuring techniques. Somewhat higher values are seen in patients with proteinuria, who are also consistently characterized by reduced glomerular filtration rate (GFR). A clear correlation is found between MAP plotted against the increased rate of microalbuminuria (%/yr) in incipient nephropathy and against fall rate of GFR (ml.min-1.mo-1) in proteinuric patients. In the natural history of renal disease, different cutoff points in MAP for start of progression are observed: greater than 95 mmHg for the start of progression of microalbuminuria and greater than 105 mmHg for the decrease in GFR. During antihypertensive treatment, there is reduction or no progression in microalbuminuria with MAP of approximately 90-95 mmHg and only a limited fall in GFR with MAP of approximately 100 mmHg. However, certain antihypertensive drugs (angiotensin-converting enzyme inhibitors) may have specific renoprotective actions, reducing microalbuminuria at rather low BP levels or even independent of BP reduction. The optimal way of monitoring BP may be by 24-h ambulatory recording.