Congenital intrarenal teratoma arising from a horseshoe kidney.

We report a case of intrarenal teratoma arising from a horseshoe kidney. A 6-day-old girl was admitted with an abdominal mass that had been noticed at 37 weeks gestation by routine ultrasonography. At 20 days of age, a tumorectomy with a right nephrectomy was performed. The pathological diagnosis was an immature teratoma. An intrarenal teratoma is extremely rare. In addition, we believe that this case represents the first case of an immature teratoma occurring in a horseshoe kidney. The diagnosis in this type of case is difficult, but we recommend that such tumors be included in the differential diagnosis.