Routine prenatal ultrasound examination of the urogenital tract is of importance in patients with posterior urethral valves (PUV), because the renal function and long-term prognosis of these patients depend on early diagnosis and subsequent therapy. Opinion is divided as to whether the often-observed association of PUV with renal dysplasia represents a primary malformation or a secondary pathology caused by recurrent infections. These aspects should have an influence on therapeutic consequences and optimal timing of therapy in order to preserve long-term renal function. The histology of kidney specimens from 13 children with PUV who underwent nephrectomy was reviewed to attempt to differentiate between primary dysplastic malformations and secondary pathologies. Clinical data were analyzed and compared with the histologic findings. The average age at nephrectomy was 29 months (range 3-158 months). Approximately 80% of the specimens showed primary dysplastic malformations (mesenchymal or fetal cartilage tissue or dysplastic glomeruli and tubuli) in the presence of well-developed renal parenchyma. All specimens showed secondary pathologies such as renal-cortical atrophy, interstitial fibrosis, and interstitial-nephritis atrophy. The histologic evidence of well-differentiated renal parenchyma in concurrence with dysplastic parenchyma makes infravesical obstruction as the only cause of renal alterations in patients with PUV questionable. This coincides with the fact that organogenesis of the kidney is terminated at the 12th gestational week and secondary renal damage is irreversible at the 20th gestational week, but prenatal urinary diversion of the upper urinary tract is feasible in the 20th gestational week at the earliest. These facts must be taken into account when considering intrauterine urinary diversion.