OBJECTIVE. Due to advances in endoscopic equipment, primary duodenal tumors are found more frequently than in the past. We performed endoscopic submucosal dissection (ESD) to diagnose and treat four non-ampullary duodenal tumors. MATERIAL AND METHODS. During endoscopic treatment, marks were placed around the circumference of the tumor and sufficient amounts of physiological saline with epinephrine were injected into the submucosal layer to elevate the lesion. An incision was made around the lesion using a long-type needle knife and the isolated lesion was resected completely "en bloc". In this procedure, a cylindrical transparent hood was attached to the endoscopy apparatus to allow for satisfactory visualization of the procedure. RESULTS. The mean age of the patients was 69 years. The patients consisted of two males and two females. Gross examination showed three flat, elevated lesions and one polypoid lesion. Tumor size ranged from 10 to 31 mm in maximum diameter. Histological examination revealed two cases of well-differentiated adenocarcinomas and two cases of tubular adenomas with severe atypia. Procedure-related complications consisting of perforation occurred in two cases and were resolved under close postoperative observation including antibiotics, use of a nasogastric tube and nil per oral feeding status. The mean follow-up period was 18 months and none of the patients experienced tumor recurrence after the treatment. CONCLUSIONS. Since tissue obtained from endoscopic biopsies can sometimes prove difficult for definitive histological diagnosis, ESD may play an important role in the management of cases appearing to border on malignancy. In addition, ESD allows for minimally invasive treatment without sacrificing the possibility of cure for duodenal carcinoma.