Extracolonic involvement of the gastrointestinal tract is extremely uncommon in ulcerative colitis (UC) and rarely found in the upper gastrointestinal tract or in postoperative cases since it typically responds to steroids. Here we report a case of UC complicated by extensive ileal inflammation that was refractory to steroids. A 20-year-old man was diagnosed with UC of typical pancolitis without ileal involvement and started treatment with pH-dependent mesalazine and oral prednisolone. Although his symptoms transiently resolved, the condition flared when the steroid dose was tapered down. Computed tomography revealed marked thickening of the ileal wall, and capsule endoscopy and balloon-assisted enteroscopy found diffuse mucosal inflammation with ulcers in the ileum. On the contrary, the inflammation in the colon and rectum was improving. Since the response to the second steroid course was inadequate, treatment with adalimumab and 6-mercaptopurine was initiated and finally achieved clinical and endoscopic remission. The investigation of small intestinal lesions is necessary in patients with UC whose clinical deterioration cannot be explained by colonic lesions.