Fourteen examples of intravenous leiomyomatosis in the AFIP files were studied. Growth beyond the uterus occurred in 10 of the 14 examples, largely in vessels in the broad ligament and in uterine and iliac veins. Metastasis to the heart occurred in 1 example. The study provides evidence that is consistent with both theories of origin of intravenous leiomyomatosis; namely, that it may arise either from the wall of veins within the myometrium or be a result of unusually extensive vascular invasion from a leiomyoma of the myometrium. Intravenous leiomyomatosis should be distinguished from a leiomyoma with vascular invasion by arbitrarily limiting the designation of leiomyoma with vascular invasion to neoplasms in which the vascular growth is a microscopic finding and confined within the leiomyoma. Criteria are given for distinguishing intravenous leiomyomatosis from endometrial stromal sarcoma, leiomyosarcoma, leiomyoma with vascular invasion, and leiomyomatosis peritonealis disseminata.