Renal function and blood pressure were assessed in 139 patients after unilateral nephrectomy. Followup ranged from 1 to 57 years, with a mean of 13.0 +/- 1.1 (standard error). Serum creatinine, creatinine clearance, beta 2-microglobulin clearance and blood pressure remained stable during followup. No significant effect of years after unilateral nephrectomy, blood pressure or cause of nephrectomy was observed on creatinine clearance. However, urine excretion of protein (correlation coefficient 0.475, p less than 0.01) and N-acetyl-beta-D-glucosaminidase (correlation coefficient 0.464, p less than 0.01) increased as a function of years after unilateral nephrectomy. Creatinine clearance tended to be low in elderly patients or patients who underwent nephrectomy at an advanced age. Our studies show that despite the late development of proteinuria and tubular injury, unilateral nephrectomy is not associated with deterioration in kidney function or elevation of blood pressure during long-term followup.