Objective: The aim of this study was to focus on fetuses diagnosed with severe hydronephrosis and correlate prenatal sonographic characteristics with postnatal outcome. Methods: Cases presenting prenatally with severe hydronephrosis (anterior-posterior renal pelvic diameter >15 mm) were collected retrospectively over a period of 11 years and divided into 2 groups: (1) isolated hydronephrosis and (2) those associated with congenital anomalies of the kidney and urinary tract (CAKUT). Results: A total of 83 fetuses comprised the study group: 35 fetuses had isolated severe hydronephrosis and 48 had associated CAKUT. The mean anterior-posterior renal pelvic diameter was 22.6 ± 8.5 mm (range 15.0-66.0 mm). The CAKUT group was associated with a significantly increased incidence of postnatal need for surgery (17.6% vs 44.2%, P =.014), dysplastic kidney (0% vs 14%, P =.023), and total abnormal outcome (52.9% vs 86%, P =.001) in comparison with isolated severe prenatal hydronephrosis. Conclusions: Severe fetal hydronephrosis has a wide postnatal clinical spectrum, which is mainly influenced by the presence of associated sonographic CAKUT findings. These clinical data have biological relevance: a genetic or environmental defect that influences multiple renal developmental processes leads to hydronephrosis but also to concomitant malformations (CAKUT) and critically influences renal prognosis. A more selective abnormal developmental process that results in isolated enlarged pelvis even to a severe extent has less influence on renal prognosis.