Seventy-nine hypertensive nephrosclerosis patients i entered a prospective randomized singleblind study to 1) establish the pattern of decay of renal function in this population and the variability therein and 2) to determine if strict diastolic blood pressure (DBP) control (<80 mm Hg) is more effective than conventional levels (90-95 mm Hg) in conserving renal function. Because of unexpected significant improvement in renal function in patients from both groups, which changed the perspectives on the course of this disease as described herein, this report is being published before completion of the trial. The selection criteria were 1) serum creatinine concentration of 1.6-7.0 mg/dl, 2) glomerular filtration rate of less than 70 ml/min/1.73 m 2 , and 3) absence of diseases (other than hypertension) known to destroy renal function. Renal function was assessed by glomerular filtration rate ([ l2S I]iothalamate clearance) and serum creatinine concentration. Before randomization, DBP was aggressively treated to reduce it to less than 80 mm Hg. Twenty-two subjects (14 in the strict DBP control group and eight in the conventional DBP control group) have been enrolled in the study for 36 months. In contrast to results from previous studies in humans and rats, renal function improved in both patient groups. Thus, irrevocable progression of renal damage after onset of renal failure from high blood pressure does not necessarily occur, and in fact, long-term improvement of renal function resulted from the effects of the study itself. The study design involving the 2-4-month initial period of aggressive DBP control at 80 mm Hg or less followed by control of DBP at less than 90 mm Hg was associated with long-term improvement in renal function. (Hypertension 1989; 13:766-772)