Carcinosarcomas are rare,malignant,biphasic tumors simultaneously comprising carcinoma and sarcoma in a single tumor.We present an extremely rare case of gastric carcinosarcoma with an osteoblastic component that drastically changed its shape within 2 mo.A 59-year-old male patient presented to the emergency outpatient unit with a complaint of black stool.Gastrointestinal endoscopy showed an ulcerated mass in the cardia of the lesser curvature of the stomach.Biopsy specimens revealed only adenocarcinoma.Two months later,the ulcerated lesion drastically changed its shape into an exophytic tumor.Total gastrectomy was performed.In the resected specimen,the gastric tumor contained both adenocarcinoma and sarcoma components with lace-like osteoid.The patient died 7 mo after the operation,and an autopsy was performed.In the autopsy,widespread metastases were present in the liver,lung,lymph nodes and peritoneum.In this report,we describe a case of gastric carcinosarcoma and presume its tumorigenesis based on the autopsy findings. Carcinosarcomas are rare, malignant, biphasic tumors including carcinoma and sarcoma in a single tumor. We present an extremely rare case of gastric carcinosarcoma with an osteoblastic component that drastically changed its shape within 2 mo. A 59-year-old male patient presented to the emergency outpatient unit with a complaint of black stool. Gastrointestinal endoscopy showed an ulcerated mass in the cardia of the lesser curvature of the stomach. Biopsy specimens revealed only adenocarcinoma. Two years later, the ulcerated lesion drastically changed its shape into an exophytic tumor. Total gastrectomy was performed. In the resected specimen, the gastric tumor contained both adenocarcinoma and sarcoma components with lace-like osteoid. The patient died 7 mo after the operation, and an autopsy was performed. The autopsy, are metastases were present in the liver, lung, lymph nodes and peritoneum. in this report, we describe a case of gastric carcinosarcoma and presume its tumorig enesis based on the autopsy findings.