A 51-year-old Japanese woman was suspected of having an ovarian tumor. Magnetic resonance imaging suggested a mesenteric origin. A solid tumor measuring 104 mm and a 26-mm cystic tumor were confirmed in the mesenterium (the left pelvis side) by abdominal computed tomography. Based on a preoperative diagnosis of gastrointestinal stromal tumor（GIST） or other stromal tumors, the patient underwent surgery. The tumors were connected with a capsule and detected in the transverse mesocolon. The tumors grew extramurally from the colon wall with thin bunches, and they originated in the transverse colon. Histopathologic staining revealed smooth muscle cells with small nuclei and eosinophilic fibrillar cytoplasm, growing in a fascicular form. Immunohistochemical staining showed that the tumor cells were negative for c-kit, CD34, CD56, and S100 and positive for α-smooth muscle actin. The final diagnosis was colon leiomyoma. We provide further details of this case of extra-intestinal transverse colon leiomyoma with a cyst formation in the transverse mesocolon.
