Colon cancer is the second most common malignancy in Korea. It is classified as superficial type, the mass type, the ulcerative type, the ulceroinfiltrative type, the diffuse infiltrative type and the unclassified type according to the colonoscopic findings. We report here on a case of colon cancer that was initially misdiagnosed as acute infectious colitis at the initial presentation. A 64-year-old man visited to Yeungnam University Hospital for watery diarrhea and lower abdominal pain. Colonoscopy revealed long segmental edematous mucosa and hyperemic mucosa with stenosis in the transverse colon. He was diagnosed as having acute infectious colitis according to the colonoscopic finding. However, two days later after colonoscopy, he visited the emergency room for hematochezia. We performed computerized tomography (CT) and obtained blood samples to find the origin of the bleeding. We found thickening of the transverse colon lumen and ascites on the CT finding and an elevated level of tumor markers; we also obtained the results of the colonoscopic biopsy that was done via colonoscopy. He was finally diagnosed as having colon cancer with carcinomatosis, a poorly differentiated adenocarcinoma.