Supernumerary kidney with ipsilateral cryptorchidism in a cat.

An 8 wk old male domestic longhair was presented with an abdominal mass and cryptorchidism. A 2 cm mass was palpable in the midabdomen. Ultrasonography confirmed a complex, septated, cystic mass adjacent and caudal to the right kidney. A normally appearing left kidney was present. Pathologic examination of the excised abdominal mass revealed it to be a kidney with an attached, normal caliber ureter. At surgery, this kidney was separate from the parenchyma of the second, cranial, right kidney. Subsequently, the second right kidney became hydronephrotic and was removed together with the cryptorchid testis and an apparently hypoplastic ureter. This is the first report of a supernumerary kidney in a cat, adding it to the differential diagnoses of abdominal masses.