The clinical course of 138 children who underwent unilateral nephrectomy and had a normal contralateral kidney at the time of nephrectomy was reviewed. The diagnosis leading to nephrectomy included obstructive uropathy in 46% of the cases, reflux or pyelonephritis in 30%, Wilms tumor in 15%, hypertension in 4%, dysplastic kidney in 2% and trauma in 2%. Mean age at nephrectomy was 7.3 years and median followup was 24.7 years. Of the 138 patients 121 (88%) are well and 17 died, including 14 secondary to metastatic Wilms tumor and 1 of renal failure. Survival of nonWilms tumor patients was similar to that of an age-matched control group. In 30 patients 24-hour creatinine clearance and 24-hour urinary protein excretion were measured. Proteinuria (greater than 150 mg./24 hours) was found in 8 of the 30 patients (27%) (p less than 0.001), renal insufficiency developed in 9 (30%) (p less than 0.0001) and hypertension occurred in 10% (p greater than 0.10). Children with an acquired solitary kidney are at increased risk for proteinuria and renal insufficiency.