A 57-year-old woman with diabetes mellitus was admitted to our hospital because of right dorsolumbar pain in October 2007. Laboratory data revealed an advanced inflammatory reaction and an abdominal CT scan showed a 5cm-sized abscess in the right ilio-psoas muscle. Percutaneous abscess drainage was performed under the diagnosis of right ilio-psoas abscess. Because she was found to have anemia, further examinations were performed. Colonoscopy revealed advanced cancer of the ascending colon and the pathological diagnosis of biopsy specimen was well differentiated adenocarcinoma. Operation was thus performed with the diagnosis of cancer of the ascending colon. Operative findings indicated that the tumor 5cm in diameter was located at the ascending colon and penetrated to the right psoas muscle with abscess formation. Because of peritoneal dissemination, right hemicolectomy (D1) combined with partial resection of the ilio-psoas muscle was performed. The pathological examination revealed that the tumor had directly invaded the abscess cavity. Colon carcinoma should be considered as a probable differential diagnosis in a patient with an abscess in the ilio-psoas muscle.