PROGNCSIS The prognosis in untreated, abnormal arteriovenous communications is uncertain. Some, as a result of infection or increasing hematoma, may lead to such serious consequences as death or gangrene of a part within a few days after their production. When these early complications do not occur, the fistulas usually run a very chronic course, except when they happen to occur between the aorta and the vena cava. There are many cases reported in which an arteriovenous or cirsoid aneurysm had been present for from ten to forty years. There are fifteen cases of such long duration reported in this paper. It is generally true that abnormal arteriovenous lesions of the arms, neck and head are better tolerated than are those of the lower extremity. As a rule, the prognosis is directly dependent on the size of the vessels involved, and on the size of the fistulous opening. Peripheral lesions, such