A malignant neoplasm characterized by a vascular proliferation which usually contains blunt endothelial cells. Erythrocyte extravasation and hemosiderin deposition are frequently present. The most frequent site of involvement is the skin; however it may also occur internally. It generally develops in people with compromised immune systems including those with acquired immune deficiency syndrome (AIDS).

A multicentric, malignant neoplastic vascular proliferation characterized by bluish-red cutaneous nodules, usually on the legs, toes, or feet, that slowly increase in size and number and spread to more proximal sites. The tumors have endothelium-lined channels and vascular spaces mixed with aggregates of spindle-shaped cells; they may remain confined to skin and subcutaneous tissue, but widespread visceral involvement may occur.