We report a case of primitive neuroectodermal tumor (PNET) arising in the transverse colonic mesentery. A 24-year-old Japanese woman was admitted to Kagoshima City Hospital with complaints of abdominal pain and sensations of abdominal fullness of 5 months' duration. On palpation, a mass the size of an infant's head was noted in the right flank. Abdominal computed tomography (CT) and ultrasonography showed a huge mass that consisted of multiple cystic components. On arteriography, a slight tumor stain appeared, with stretched and displaced tributaries of the right colic and middle colic arteries. Barium swallow examination demonstrated that the ascending colon was shifted to the right and small intestine to the left. We performed an en-bloc resection of the tumor in the transverse colonic mesentery, including the ascending colon, proximal jejunum (20 cm in length), and greater omentum. The resected tumor was 12 x 10 x 7 cm in size, 590g in weight, elastic soft in consistency, and multicystic. Histologically, the specimens showed a sheet-like proliferation of spindle-to-polygonal cells, and focally, the tumor formed rosette structures. Immunohistochemically, the tumor cells were positive for neuron-specific enolase (NSE) and mic-2. EWS-FLI1 chimeric mRNA was detected by reverse transcriptase-polymerase chain reaction (RT-PCR). Based on the above findings, we finally diagnosed the tumor as PNET of the colonic mesentery. There has been no recurrence for 20 months after operation. PNET arising in the mesentery is very rare, and we distinguished PNET from other tumors by immunohistochemical examination and by demonstration of the presence of EWS-FLI1 chimeric mRNA in the tumor.