A 64 year-old woman with steroid-dependent immune thrombocytopenia developed anemia. Esophagogastroduodenoscopy revealed the presence of a tumor, which was diagnosed to be diffuse large B-cell lymphoma, in the second portion of the duodenum. 18F-fluorodeoxy glucose positron emission tomography showed an increased uptake mass in the pelvic cavity as well as in the duodenum. Though the duodenal tumor disappeared after 4 cycles of chemotherapy, the pelvic mass did not shrink in size. As a result, laparoscopic resection of the pelvic tumor was performed and the tumor was histologically diagnosed to be a gastrointestinal stromal tumor. Subsequently, the patient was treated with 2 more cycles of the chemotherapy. Eventually, thrombocytopenia completely resolved.