Although retroperitoneal or psoas abscess is an unusual clinical problem, the insidious and occult characteristics of this abscess sometimes cause diagnostic delays, resulting in considerably high morbidity and mortality. In particular, psoas abscess caused by perforated colon carcinoma is uncommon. We report a case of psoas abscess caused by a carcinoma of the cecum. A 72-year-old Japanese woman was admitted to our hospital, with pain in the right groin and buttock. The pain had appeared 6 months before admission, and the symptoms had then been relieved by oral antibiotics. On March 25, 1999, inflammatory signs in the right buttock indicated localized cellulitis, and incision and drainage was performed at a local hospital. The patient was referred to our hospital on the same day. On admission to our hospital, computed tomography (CT) scan revealed a thick right-sided colonic wall and enlargement of the right ileopsoas muscle. Barium enema and colonofiberscopy revealed an ulcerated tumor occupying the entire circumference of the cecum. A retroperitoneal abscess and fistula had been formed by the retroperitoneal perforation of cecum carcinoma: surgical resection was performed after remission of the local inflammatory signs. Operative findings indicated that the cancerous lesion and its surrounding tissues were firmly attached to the right iliopsoas and major psoas muscle, and en-bloc resection, including adjacent muscular tissue, was performed. The fact that carcinoma of the colon could be a cause of psoas abscess and cellulitis in the gluteal region should be considered when an unexplained psoas abscess is diagnosed.