Background: Gastrointestinal stromal tumors (GIST)comprise a majority of tumors previously diagnosed as gastrointestinal leiomyomas, leiomyoblastomas, and leiomyosarcomas. Although GIST may be identified by light microscopy, pathologists commonly employ a panel of immunohistochemical markers to confirm the morphological impression including anti-CD34, smoothmuscle actin, desmin, S100, and CD-117. However, in CD-117 negative cases, it becomes difficult to diagnose GIST. This is of great importance, since the use of imatinib mesylate has led to a dramatic improvement in survival rates of GIST patients besides improved quality of life. Case Report: We report the case of a 67-year-old male patient diagnosed as having chemotherapy-resistant metastatic leiomyosarcoma. We reviewed the specimen for a possible diagnosis of c-kit negative GIST. Plateletderived growth factor receptor-alpha (PDGFRA) immunohistochemical stain was recommended. The specimen was positively stained for PDGFRA. Imatinib mesylate(400 mg/d) was started. The patient showed an excellent response after receiving imatinib treatment which was documented with 18 fluoro-deoxyglucose positron emission tomography and computed tomography (18F-FDGPET/CT) prior to and after treatment. Conclusion: The importance of diagnosis of c-kit negative GIST is emphasized while stressing the importance of PDGFRA staining besides PET/CT response evaluation.