Forty-eight boys with posterior urethral valves were reviewed to assess the impact on prognosis of prenatal ultrasonography, which had been the method of diagnosis in 17 of them; the remaining 31 presented postnatally. Renal function before treatment was mildly to moderately impaired in 59% of those presenting prenatally, 42% presenting before 3 months of age and 25% of those first presenting when older than 3 months. All but one of the infants diagnosed prenatally had clinical signs and/or elevated serum creatinine in the early neonatal period. Eight had associated maternal oligohydramnios, of whom 6 presented prenatally and 2 postnatally: this factor was associated with a poor prognosis, with renal impairment persisting in 6 of these patients after treatment. Renal function did not relate consistently to the presence or otherwise of vesicoureteric reflux (unilateral or bilateral). Twelve of the patients who presented postnatally had normal fetal ultrasonography late in pregnancy, of whom 11 had upper tract dilatation when investigated. Six of these patients who currently have impaired renal function did not have maternal oligohydramnios, and it is argued that antenatal detection of the condition would have been beneficial in less than 20% of patients presenting postnatally with posterior urethral valves.