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Background: Cotyledonoid dissecting leiomyoma is a rare variant of uterine smooth muscle tumor with an unusual growth pattern that shows intramural dissection within uterine myometrium and often a placenta-like appearance in its extrauterine components., Case Presentation: We present a unique case of cotyledonoid dissecting leiomyoma with adenomyosis. A 40-year-old Japanese female presented with prolonged menorrhagia and severe anemia. She had a pelvic mass followed-up for 6 years with a diagnosis of leiomyoma. However, increase in tumor size and cystic changes with hemorrhage were found by magnetic resonance imaging, and total abdominal hysterectomy with bilateral salpingectomy was performed. Macroscopically, the placenta-like exophytic mass protruding from the posterior uterine wall was composed of multiple nodules containing numerous hemorrhagic cysts. The mass showed continuity as a white multinodular dissecting mass infiltrating the posterolateral myometrium. Microscopically, both extra-and intrauterine portions of the mass were composed of nodules that contained swirled neoplastic smooth muscle cells with marked hyalinized degeneration, as observed in cotyledonoid dissecting leiomyomas of conventional type. In addition, numerous non-neoplastic glands of endometrial type surrounded by abundant endometrium-like stromal cells and non-neoplastic smooth muscle cells were found in the tumor, suggesting that it involved a part of concomitant adenomyosis originating from the nontumoral myometrium., Conclusions: Thus far, over 30 cases of cotyledonoid dissecting leiomyoma have been reported, none of which have described the presence of adenomyosis within the tumor. The present case suggested that cotyledonoid dissecting leiomyoma might have a unique clinical presentation involving concomitant uterine adenomyosis. It is critical for pathologists, gynecologists, and radiologists to be cognizant of cotyledonoid dissecting leiomyoma variants for timely and appropriate diagnosis and treatment.