A 67-year-old woman presented with a history of upper abdominal pain and weight loss. Physical examination revealed a lump in the right lower quadrant. She had undergone esophagogastroduodenoscopy at another hospital ten years ago, which showed a 15-mm elevated lesion in the duodenal bulb. The patient had not undergone further examinations or received treatment during the 10 years. Esophagogastroduodenoscopy conducted in our hospital revealed an enlarged tumor that was difficult to assess on the whole image. The tumor was diagnosed as a well-differentiated adenocarcinoma based on a biopsy specimen. Enhanced computed tomography revealed a hypervascular duodenal tumor with liver and lymph node metastases. The patient was treated with capecitabine plus oxaliplatin for the duodenal cancer. Lymph node metastases increased markedly after 2 courses of chemotherapy. The patient died 3 months after the initiation of chemotherapy. The natural history of sporadic non-ampullary duodenal epithelial tumors remains to be fully elucidated due to the low incidence rate. This case suggests that sporadic non-ampullary duodenal epithelial tumors have a biological potential for invasive malignancy.