Granulocytic sarcoma (GS) is an uncommon and localized extramedullary tumor composed of immature granulocytic cells. Most GS reported in large series were not associated with overt acute myelogenous leukemia. Gastric perforation occurred during prednisolone therapy in a 72-year-old Japanese male with a four-month history of a myelofibrosis-like state. Subtotal gastrectomy was performed for a suspected gastric ulcer perforation. Gastric histologic, immunohistochemical and cytochemical examination revealed diffuse infiltration by sheets of myeloblasts and promyelocytes with scant or moderately abundant cytoplasm including a few eosinophilic myelocytes. Bone marrow study done in one month after the operation disclosed refractory anemia with excess of blasts (RAEB). Leukemic transformation occurred two months later, and a subcutaneous tumor appeared on the forehead. The forehead tumor predominantly consisted of myeloblasts without evidence of maturation. Both the stomach and forehead tumors were examined immunohistochemically with a panel of monoclonal antibodies (LCA, L26, MT1, UCHL1, OPD4, LN-1, LN-2, LN-3, MB1, Leu-M1, PM) and polyclonal antibodies (lysozyme, alpha 1-antitrypsin, alpha 1-antichymotrypsin, S-100 protein, lactoferrin), as well as naphthol-ASD-chloroacetate esterase staining to investigate and characterize the reliable marks for GS, and the patient was diagnosed as GS. We found that gastric GS may occur in a myelofibrosis-like state followed by RAEB of myelodysplastic syndrome and that naphthol-ASD-chloroacetate esterase staining and immunohistochemical detection of MT1, lysozyme, and alpha 1-antitrypsin were the most reliable markers for confirming the diagnosis of GS.