The subject of abnormal arteriovenous communications is so large that it seems inadvisable to discuss it in one paper; therefore I shall attempt to present it in a series of papers including: (1) a report of cases; (2) the origin and nature of arteriovenous aneurysms, cirsoid aneurysms and simple angiomas; (3) the effects of arteriovenous aneurysms on the heart and blood vessels and other structures, and (4) the treatment of arteriovenous and cirsoid aneurysms. The material for these papers is derived mainly from a study of the author's own cases of arteriovenous communications and from experimental work done in the laboratories of the Johns Hopkins University and the University of Cincinnati. It is further supplemented by a careful study of all the cases recorded in the Johns Hopkins Hospital and by an extensive study of the literature on the subjects of arteriovenous aneurysms, cirsoid aneurysms, racemose aneurysms, aneurysms by anastomosis,