Both subcutaneous and hepatic hemangiosarcomas were induced in mice exposed to vinyl chloride monomer, and for the first time, the ultrastructure of hemangiosarcomas related to vinyl chloride exposure has been described. The subcutaneous hemangiosarcoma developed in a mouse's ear 29 weeks after exposure to 10 ppm vinyl chloride for 4 weeks, and the hepatic hemangiosarcoma was found in a mouse's liver at necropsy 56 weeks after exposure to 600 ppm vinyl chloride for 4 weeks. Both tumors showed a localized, cystic nodular appearance with a dark red tone. Histologically, an angiomatous architecture, the presence of neoplastic mesenchymal cell aggregates, focal necrosis, and hemorrhagic foci were common. Ultrastructurally, two neoplastic cell types, a mesenchymal and a well-differentiated endothelial cell type, were identified in both tumors. In addition, in the hepatic tumor, a pericyte-like neoplastic cell was also present. The incidence of hemangiosarcoma was much lower than that of lung tumors (alveologenic tumor) in mice exposed to vinyl chloride at the same concentrations for the same duration. This study suggests that there is no basic difference in ultrastructure between the vinyl chloride-induced hemangiosarcomas and hemangiosarcomas not related to this chemical exposure.