In two patients with malignant germ cell tumors angiosarcoma developed through two apparently different mechanisms. In one case the angiosarcoma probably developed as a complication of therapeutic radiation, since radiation changes were demonstrated in tissue adjacent to the neoplasm and since the angiosarcoma was not associated with elements of germ cell tumor. The absence of associated germ cell elements does not support the development of the angiosarcoma from a teratoma. In the second case, however, it is likely that the angiosarcoma developed as a result of malignant change within teratomatous foci, since angiosarcomatous elements were intermingled with teratomatous elements and the patient's primary germ cell tumor contained malignant and atypical teratomatous elements as well as prominent vascular proliferation. Malignant change within teratomatous components of germ cell tumors is a phenomenon of increasing importance in this era of effective chemotherapy for germ cell tumors. The development of angiosarcoma as a potential complication of testicular carcinoma has not been reported previously.