We received requested follow-up information from 105 (73%) of our 144 kidney donors who had undergone unilateral nephrectomy 10 to 20 years previously. Five donors had died of unrelated causes 6 or more years postoperatively. Studies in the remaining 100 donors showed that the current mean serum creatinine concentration was 1.2 mg/dl and the mean 24-hour urinary protein value was 89 mg. Hypertension (defined as 160 mm Hg or more systolic, 95 mm Hg or more diastolic, or both) was present in 19% of the donors. In a subgroup of 66 donors who had had serial serum creatinine determinations, the renal function, as estimated on the basis of these serum creatinine values, had not deteriorated with time. Thus, we consider unilateral nephrectomy in this group of patients relatively safe. Subsequent evaluation will be necessary to ascertain whether these findings prevail.