The ultrasonographic detection of renal anomalies may modify obstetric management and facilitate pediatric care of the newborn. We performed prenatal differential diagnosis of an isolated unilateral cystic renal mass (71 × 74 × 82 mm) in a pregnant woman at 26 weeks of gestation. No other abnormalities were detected by ultrasonography, except for polyhydramnios. Repeated percutaneous cyst aspirations were required because of the increasing risk of vital organ damage. Postnatal nephroureterectomy was performed. Anatomopathologic analysis led to the diagnosis of segmental renal dysplasia, which could not be included in any of the four groups of Potter's classification of cystic renal dysplasia.