This report describes an unusual case of large cystic adenomyoma in a 37 year-old woman. Cystic degeneration is often seen in a leiomyoma, but has not been reported so far in adenomyoma. TVUS and MRI were carried out in the diagnosis. TVUS demonstrated the cystic nature of the mass and its characteristic hemorrhagic pattern, and MRI gave more detailed information about the lesion and its content. The patient underwent surgical resection of the mass, and diagnosis of cystic adenomyoma was confirmed pathologically.