A 38-year-old man was admitted with a high fever, sore throat, and right upper quadrant pain. Nine months before his admission, he had undergone right hemicolectomy under the impression of intestinal lymphoma. But there had been no evidence of lymphoma on microscopic examination. Under the postoperative diagnosis of inflammatory bowel disease, corticosteroid therapy was tried without response. On the follow-up colonoscopic examination, an ovoid ulcer, with convergence of the surrounding mucosal folds at the descending colon and an irregularly shaped ulcer at the ileocolic anastomotic site, were found. The colonoscopic diagnosis was Behcet's colitis. After pathologic slides of biopsy and surgical specimens obtained from the palatine tonsil and colon were reviewed, the diagnosis of polymorphic reticulosis was made. The patient received anticancer chemotherapy, including cyclophophamide and glucocorticosteroid. To date, colonic involvement of polymorphic reticulosis has not been reported. Because of the similarity of the colonoscopic findings to those of Behcet's colitis, polymorphic reticulosis should be included in the differential diagnosis of inflammatory bowel disease. We assume that this is the first case of polymorphic reticulosis involving the colon with characteristic colonoscopic findings.