To assess the effects of unilateral nephrectomy, we evaluated renal function and hypertension in kidney donors who had had nephrectomies 10 years ago or more and siblings who had not had nephrectomies. No statistically significant difference was found between the prevalence of hypertension in donors and siblings. Serum creatinine concentrations were 20% higher in donors and creatinine clearances, 20% lower than corresponding values in siblings. Twenty-four-hour urinary protein excretion increased in all donors after nephrectomy and was more marked in men than women. Of the 38 donors, 12 excreted more than 150 mg/24 h of urinary protein, but only 2 excreted more than 300 mg/24 h. The presence of proteinuria did not correlate with the presence of hypertension, level of renal function, or time since nephrectomy. We conclude that, with the exception of mild proteinuria of unknown clinical significance, unilateral nephrectomy is not associated with adverse effects on kidney function.