A surgical case of leiomyosarcoma arising from the ascending colon, presenting as acute suppurative peritonitis, is herein described. A 70-year-old woman complaining of lower abdominal pain presented to our clinic on October 12, 1994. She was admitted with a tentative diagnosis of peritonitis. At emergency laparatomy, purulent intraabdominal fluid was present, and a fist-sized mass was seen in the ascending colon just proximal to the hepatic flexure. A right hemicolectomy was thus performed based on a diagnosis of perforating colon cancer. The histologic findings were consistent with leiomyosarcoma with abscess formation in and around the tumor. Five mitotic figures per field were observed at 10x magnification. Immunohistochemical studies revealed immunoreactivity for alpha-smooth muscle antigen (alpha-SMA), vimentin, and desmin. After reviewing the clinicopathologic characteristics of colon leiomyosarcoma as described in 78 Japanese cases and 70 cases from the foreign literature, we thus propose that colon leiomyosarcoma frequently arises from the transverse colon. In addition, our case also represents the only reported case in Japan in which an adult patient underwent a successful operation for perforated leiomyosarcoma of the colon.