The authors report the case of a male neonate presenting with a heterogeneous cystic mass in the right renal area with no identifiable renal unit. The diagnosis of right perirenal extracapsular uro-haematoma secondary to posterior urethral valve was proposed on the basis of the antenatal clinical history. The late discovery at 39 weeks of amenorrhoea (pregnancy not followed) of a foetus with trabeculated bladder associated with left ureteropelvic dilatation, and a cystic mass occupying the right lumbar fossa was highly suggestive of right urinoma secondary an infravesical obstacle. In this case, vaginal delivery was probably responsible for intracystic bleeding of the urinoma. The postnatal morphological assessment including ultrasound, retrograde cystourethrography and MRI confirmed the presence of a posterior urethral valve complicated by right uro-haematoma displacing the right kidney and left ureteropelvic dilatation. This lesions corresponds to a collection of urine and blood in the right perirenal space due to caliceal or parenchymal perforation secondary to raised urinary tract pressure above an obstacle. Subcapsular or extracapsular perirenal urinoma, sometimes detected in neonates with posterior urethral valve, reflects protective decompression of the urinary tract (safety valve effect), as in our case. The neonatal diagnosis of a heterogeneous renal cystic mass is sometimes difficult (polycystic renal dysplasia, cystic lymphangioma, cystic neuroblastoma, adrenal haematoma). In some cases, the antenatal history may suggest the diagnosis of perirenal urohaematoma secondary to infravesical obstruction.