The majority of Wilms' tumors present in young children as an intraabdominal mass of renal origin. The use of multimodal treatment protocols according to internationally recognized treatment guidelines has resulted in extremely high cure rates even in the setting of advanced disease. By comparison, Wilms' tumors in older patients or Wilms' tumors located entirely outside the kidney are extremely rare. Because of the rarity of these tumors and the lack of established treatment guidelines specific to these lesions, older patients with atypical Wilms' tumors are at risk for under treatment and poorer outcomes. We report the case of an 18-year-old man with a Wilms' tumor arising within a mediastinal teratoma and discuss the management of this extremely rare lesion.