Granulocyte colony-stimulating factor (G-CSF)-producing malignancies are thought to be rare and associated with advanced disease and poor prognosis. Here, we report on a 77-year-old patient with G-CSF-producing gastric cancer. We observed this patient from the stage prior to the diagnosis of gastric cancer when leukocyte count was normal to the stage of advanced disease associated with remarkable leukocytosis. Immunohistochemical analysis demonstrated G-CSF expression in the advanced-stage, poorly differentiated adenocarcinoma, but not in the early-stage, well-differentiated adenocarcinoma. G-CSF receptor was not detected to be expressed in the advanced-stage tumor. Based on these results it appears that a histological change in the tumor may influence G-CSF production and the concomitant rapid progression in gastric cancer.