Renal function was studied in 24 children with chronic hydronephrosis and renal insufficiency. The follow-up period after reconstructive surgery was 1 to 12 years. Glomerular filtration rate (GFR) was assessed by the clearance of endogenous creatinine or inulin. Effective renal plasma flow was assessed by the clearance of PAH. Reconstructive surgery was performed during the first year of life in 12 out of 24 patients, between one and two years of life in 6 patients, and after two years of life in 6 patients. Three different patterns of evolution could be observed after relief of obstruction: (1) An improvement or a normalization of renal function only occurred in patients operated upon before one year of life. (2) A stabilization of renal function without normalization was observed in patients operated upon between one and two years of life. (3) A pro-gressive deterioration of renal function towards terminal renal failure was observed in five out of six patients operated upon after two years of age. This deterioration could not be explained by recurrence of detectable urinary tract infection or urinary stasis. The changes in GRF in four patients with a solitary kidney followed the same pattern. We conclude that it is essential to correct severe chronic hydronephrosis associated with renal insufficiency before one year of age if a lasting improvement of renal function is to be expected.