An unusual instance of primary leiomyosarcoma of the left long saphenous vein, possibly the tenth recorded case, is presented. Leiomyosarcoma of large veins appears to be a distinct entity, in contrast to the benign counterpart, leiomyoma. The origin of the tumor from the vessel wall, its intraluminal growth, and its ultrastructure are demonstrated. Incidence, sex and age distribution, fate and therapy of patients who have leiomyosarcomas of the large veins gleaned from reported cases in the literature are included.