A gastric carcinoma associated with marked leukocytosis is reported. The case involved a 70-year-old male who was admitted to Akita Kumiai General Hospital because of general fatigue and vertigo in October, 1989. A palpable, hard, uneven mass was found in the epigastrium. Further, an upper gastrointestinal series revealed a large filling defect in the gastric body. Also, an endoscopic examination disclosed an uneven mass, and specimens that were biopsied showed an adenocarcinoma. The white blood cell (WBC) count was 45,800/microliter and had a 96% presence of neutrophils, and an examination of the bone marrow showed excessive myeloid hyperplasia. The WBC gradually increased, reaching up to 67,800/microliter. Finally, an EIA assay of the serum colony stimulating factor (CSF) disclosed a marked increase. After a gastrectomy, the WBC promptly decreased to the normal range and the CSF activity in a supernatant of a tumor-cell-conditioned medium showed a higher value. Thus, this tumor was diagnosed as being a CSF-producing gastric carcinoma.