A 34-year-old man who had a history of ulcerative colitis (UC) was admitted to our hospital with complaints of arthralgia, erythema nodosum, recurrent oral aphthous ulcers and bloody stools. A colonoscopy revealed multiple aphthous ulcers on his cecum and colon and also revealed a transmural ulcer on his rectum consistent with a diagnosis of UC. The patient was HLA-B51 positive. Based on clinical evidence [recurrent oral ulcers, skin lesions (erythema nodosum), positivity for pathergy test] this patient was diagnosed as having Behçet's disease with gastrointestinal involvement. We describe this rare case of Behçet's disease with colitis and discuss the difficulties in making a differential diagnosis between Behçet's disease and the inflammatory bowel diseases.