In 64 children (age 1-15, mean 7.3 years) with unilateral idiopathic hydronephrosis, measurements were made of glomerular filtration rate (GFR), separate glomerular filtration rate (SGFR, measured as 51Cr-EDTA clearance combined with renography) and renal concentrating capacity. Onset symptoms were urinary tract infection in 34 children and abdominal pain in 25, while 5 were asymptomatic. Surgery was performed soon after the first investigation in 34 children and during the follow-up period in 14, and 16 children received only conservative treatment. The mean follow-up time was 4.4 years. At the time of diagnosis the total GFR and the SGFR of the affected kidney were normal or almost normal in all the children (respective means 108 +/- 16 and 50 +/- 12 ml min-1 X 1.73 m2(-1)). The total GFR remained normal at follow-up, but SGFR became subnormal in two conservatively and two surgically treated children. The concentrating capacity was initially lower in the children with, than in those without urinary tract infection. The former values had increased significantly at follow-up examination. The study indicates that the parenchymal function in unilateral idiopathic hydronephrosis in children more than 1 year old usually is normal, but may deteriorate due to urinary tract infection. Such infection should be carefully searched for and treated. Since very few of these children seem to have pelvic obstruction, surgery seldom is primarily necessary.