A case of a large retroperitoneal tumor in a previously asymptomatic twenty-two-year-old white female is presented. A review of the literature confirms the rarity of this tumor. Its histologic and embryologic derivation as well as its subtle and bizarre method of presentation are discussed. The cause of such lesions is debatable, but primary cure can be accomplished by meticulous excision of the lesion or marsupialization. This seldom seen neoplasm must enter into the differential diagnosis of all retroperitoneal masses.