An autopsy case of carcinosarcoma of the liver producing granulocyte-colony stimulating factor (G-CSF) is reported. The patient, a 74-year-old Japanese man, presented with multiple liver masses. His serum G-CSF was elevated to 286 pg/mL and a marked leukocytosis of 19 100/microL was observed. The patient had a rapidly aggravated clinical course and died 57 days after admission. Autopsy revealed a liver carcinosarcoma composed both of hepatocellular carcinoma (HCC) and sarcomatous elements immunoreactive with alpha-smooth muscle actin and desmin. Immunohistochemistry revealed positive staining of G-CSF in the cytoplasm of HCC, whereas none of the spindle cells was positively stained. Production of G-CSF was also confirmed by enzyme-linked immunosorbent assay, using the frozen tumor tissue taken at the autopsy. Similar to the majority of G-CSF-producing tumors in the literature, only the epithelial elements of the present case were immunopositive for G-CSF. Although a monoclonal origin of carcinosarcomas has generally been proposed, heterologous differentiation from a single clone might lead to the production of G-CSF only in the epithelial element in the present case. It is suggested that G-CSF was associated with the high-grade transformation of the epithelial elements, as well as the reported phenomenon of conventional HCC producing G-CSF.