We examined the effect of long-term blood pressure control on renal function in 41 patients with refractory hypertension by using minoxidil, sympathetic suppressants, and diuretics continuously for 6 months to 7 1/2 years. In 15 of 32 patients with benign hypertension, the serum creatinine concentration increased by more than 1 mg/dL, with nine of 15 requiring hemodialysis. Analysis of 1/serum creatinine versus time plots indicated that use of minoxidil delayed the onset of end-stage renal failure in some patients for up to 6 years. In the remaining 17 patients with benign hypertension, renal function remained stable with no decreases greater than 2 mg/dL. Four of nine patients presenting with malignant hypertension had marked and sustained improvement in renal function, although three initially required hemodialysis. The mean serum creatinine concentration in these four patients fell from 9.7 to 2.9 mg/dL. Thus, impressive renal functional improvement may occur with minoxidil use in some patients with malignant hypertension.