The authors present an ultrastructural study of two cases of Stewart-Treves syndrome in elderly women. Under light microscopy, the dermal and hypodermal proliferations showed vascular features which were more clearly preceptible in the second case than in the first. Ultrastructural studies showed, in the first case, clefts containing erythrocytes and surrounded by neoplastic cells which were linked by zonulae adherentes. A rudimentary form of basal membrane was also noted, with an exterior lining of pericyte-like cells. In the second case, typical capillary vessels were bordered by endothelial cells containing a few pinocytotic vesicles and microfilaments, in contact by zonulae adherentes. The walls of these vessels consisted of a clearly defined membrane lined with pericytes, and constituting, in some areas, an anastomotic network due to the swelling of the endothelial cells. Ultrastructural study suggests an angiosarcomatous origin of these lesions. Finally, the article includes a review of the literature.