A 50-year-old man with a hypervascular submucosal tumor in the second part of the duodenum presented melena during chemotherapy with Irinotecan, S-1, and Bevacizumab for peritoneal metastasis of rectal cancer. Esophagogastroduodenoscopy showed a bleeding ulcer on the submucosal tumor. The ulcer healed after discontinuation of Bevacizumab. Laparoscopic surgery was performed to excise the submucosal tumor. The mass measuring 20 × 16 mm in size was diagnosed as a very-low-risk gastrointestinal stromal tumor (GIST) histopathologically. An ulcer on GISTs is associated with high-grade malignancy. Although the GIST was very low risk in our case, an ulcer formed. The cause of the ulcer was related to administration of Bevacizumab, an angiogenesis inhibitor, for the hypervascular tumor. Caution must be exercised when Bevacizumab is given to patients with GISTs.
