It is well known that hypertension (HT) is frequently accompanied with diabetic nephropathy (DN) and that HT contributes to progression of DN. Thus, proper anti-hypertensive therapy is required in hypertensive patients with DN. However, there is so far no consensus of optimal blood pressure (BP) level to maintain the renal function in these patients. In order to evaluate the optimal BP level in the patients with renal insufficiency, we investigated the relation between BP and renal function in 15 DN patients with HT (aged 56.9 +/- 11.7 years at the first medical examination; 6 male and 9 female, total 117 patient-years) and 20 patients with hypertensive nephropathy (aged 44.3 +/- 13.0 years at the first medical examination; 17 male and 3 female, total 207.5 patient-years) as the control, who receive antihypertensive therapy for more than 4 years as outpatients at the second department of internal medicine of Tohoku University Hospital between 1974 and 1990. During this period 7 patients with DN came to receive hemodialysis therapy 2 to 6 (average 3.8 +/- 1.3) years after the first medical examination. As a result, in patients with hypertensive nephropathy, there was a tendency to show that the lower the mean BP was, the better the renal function. On the contrary, in DN patients there was an optimal mean BP (MBP) range; i.e, when MBP was controlled in this range, the deterioration rate of renal function was delayed, while deviation of MBP from this range made the renal function worse (p less than 0.01). However, this range varied with the serum creatinine (SCr) concentration level.(ABSTRACT TRUNCATED AT 250 WORDS)