We report a case of leiomyosarcoma originating from the second portion of the duodenum and associated with von Recklinghausen's disease. A 62-year-old man was admitted to our hospital complaining of abdominal pain and melena. A 5 cm tumor in the descending part of the duodenum was detected by hypotonic duodenography, CT, ultrasonography and endoscopy. Angiography revealed the tumor to be supplied by the mesenteric artery of the transverse colon. A segmental resection of the distal part of the duodenum was performed, preserving the head of the pancreas. Histologically, the tumor was a leiomyosarcoma, and the surgical margin was free of tumor cells. This surgery is safer than pancreatoduodenectomy, and is appropriate for treating non-epithelial tumors in the distal part of the duodenum in the absence of invasion of the head of the pancreas and regional lymph node metastasis. When carrying out the procedure, it is essential to ligate the branch of the pancreaticoduodenal vessels as close as possible to the duodenal side in order to preserve the blood supply to the pancreatic head.