OBJECTIVE To document the functional outcome of patients with prenatally detected posterior urethral valves (PUV) in the second decade of life, and to evaluate the possible impact of prenatal diagnosis on the long-term outcome of this condition. METHODS We analysed the functional outcome of 25 patients with prenatally detected PUV born between 1984 and 1996, whose mean (range) age at follow-up was 17.7 (10-23) years. The findings were compared with those in 17 patients (mean age 16.1 years) who had presented clinically to our unit during the same period. The duration of follow-up in both groups was >or=10 years. Late outcomes were also compared with published data for PUV. Outcome measures included; death, incidence of end-stage renal failure (ESRF), age at transplantation and the most the recently available plasma creatinine level in untransplanted patients. We also examined any possible association between functional outcome and early predictors, including nadir plasma creatinine level at <1 year and vesico-ureteric reflux (VUR). RESULTS Three patients died (12%), two as neonates and one aged 3 years. Of five patients who had been shunted in utero, four died or developed early-onset renal failure. In the 23 prenatally detected patients who survived the neonatal period, four (17%) had a renal transplant at a mean (range) age of 6.5 (3.0-12.0) years. Of 19 patients with prenatally detected PUV who had not been transplanted in the first 12 years of life, only one (5%) developed new-onset ESRF at 10.0-23.4 years whilst 11 (58%) of these patients had normal creatinine values. In the untransplanted patients there was a statistically significant correlation between age and plasma creatinine level, but no correlation between late functional outcome and nadir creatinine in the first year of life, or bilateral VUR. CONCLUSIONS Prenatal diagnosis had little impact on mortality or ESRF in the first decade of life. This appears to be largely predetermined by renal dysplasia and the severity of intrauterine obstruction. However, the functional outcome of patients with prenatally detected PUV aged 10-23 years was considerably better than published long-term data and the outcome of clinically presenting patients in our study. These findings suggest that the long-term prognosis of PUV of intermediate severity might be improved by prenatal diagnosis.