Adenomyotic cyst is very rare disease, their sizes are mostly lesser 5mm. The intrauterine adenomyotic cyst may arise from progressive expansion of cyst due to progressive menstrual bleeding. Authors experienced a case of large adenomyotic cyst within myometrium occuring in a l9-year-old woman, and who was accompanied with endometriosis. The cyst was about 3 x 3em sized, and had chocolate colored thick viscous contents, We experienced one case of adenomyotic cyst which was thought to be degenerated uterine myoma, so we report the case with a brief review of the concerned literatures.