A case of ulcerative colitis complicated with gastric and duodenal lesions is reported. The patient was a 17-year-old male who was admitted with bloody diarrhea and abdominal pain. Based on the endoscopic and histological findings of the colon, a diagnosis of ulcerative colitis was made. Upper gastrointestinal endoscopy showed multiple erosions and granular changes in the antral greater curvature of the stomach and descending portion of the duodenum. Histological examination of the stomach and duodenum revealed marked inflammatory cell infiltration and crypt abscesses. Clinically, the gastric and duodenal lesions did not respond to antiulcer drugs, but were alleviated by steroid. It was concluded that the pathogenesis of the gastric and duodenal lesions in this patient was similar to that of the colonic lesions of ulcerative colitis.