Whether extensive ablation of renal mass in humans leads to progressive glomerulosclerosis, proteinuria, and hypertension, as it does in animal models, is a matter of controversy. We have studied kidney function in six patients who underwent enucleation of a renal cell carcinoma in a solitary kidney. Four patients had previously had a nephrectomy. The two others each had one atrophic, nonfunctioning kidney. Serum creatinine levels before surgery were within the normal range (mean, 99.9 mumol/L [1.13 mg/dL]). Two weeks after tumor enucleation, creatinine levels were significantly higher than the preoperative values (mean, 124.6 mumol/L [1.41 mg/dL]). The follow-up period varied from 10 to 23 months. In all patients, kidney function improved during the following months. Serum creatinine levels nearly reached preoperative values in all patients (mean, 105.2 mumol/L [1.19 mg/dL]). None of the patients showed a progressive deterioration in renal function or proteinuria. We found a modest increase in blood pressure in two patients who had been normotensive before surgery. In conclusion, tumor enucleation in a solitary kidney did not cause significant renal injury to the remnant kidneys in our patients, at least in the short term.