Two cases of lymphangiosarcoma arising in a chronic lymphedematous extremity following mastectomy (Stewart-Treves syndrome) were reported with not only standard histology, but also special study on the ultrastructure and immunohistology. These cancers developed as a result of chronic lymphedema of the involved limb following mastectomy 15 and 16 years ago, respectively, for breast carcinoma. Immunohistologic and electron microscopic examinations proved that this tumor originates in the vascular endotheliocytes, even though they are clinically chronic lesions apparently derived from the lymphatic vessels. This sarcoma develops multicentrically in an edematous arm and spreads out rapidly, so prognosis is quite discouraging in those patients. They need to be radically surgically treated without hesitation; hence, amputation, not limb-saving surgery, should be indicated.