The idea as to etiology which now seems most generally accepted is that blackwater fever occurs almost solely in those who have resided for considerable periods of time in districts where malignant tertian malaria is very prevalent and intense and who have repeatedly suffered from such malarial attacks. Rarely blackwater fever may be connected with benign tertian infections or exceptionally with quartan ones. As a result of the damage done the patient by the malarial attacks there is a tendency on the part of his red cells to haemolysis which may be due to the production of a hypothetical autolysin or to anaphylactic sensitizations, as has been suggested recently. Malaria is the predisposing cause and the exciting cause may be any of a number of different factors capable of lowering body resistance such as the occurrence of another malarial attack, the administration of quinine, particularly of the acid salts of quinine in rather large doses, refrigeration, as brought about by one’s clothes becoming wet and then later subjected to the chilling influence of a sea breeze, to excessive fatigue or dietetic or alcoholic excesses. Quinine administration, particularly if associated with refrigeration, is the most common exciting factor. As regards the association of malaria and blackwater fever Stephens, in a study of 390 cases of blackwater, found that 73% of the cases showed malarial parasites on the day preceding the haemoglobinuria, 47.5% on the day of the attack and 23% on the day following the appearance of the dark urine. Other workers give higher figures as 95, 70 and 20%. Where one utilizes the methods of examining for increased percentage of large mononuclears or for melaniferous leucocytes, in those cases not showing malarial parasites, the percentage of evidence of malaria is greatly increased. It is necessary to understand that a small percentage of cases diagnosed as blackwater fever do not show evidences of malaria at autopsy and cases are recorded where blackwater has attacked persons who had never had malarial fever, such instances, however being exceptional. _The Quinine Theory._—This idea as to the causation of blackwater fever first originated with Veretas, in Greece, in 1858. Later Tomaselli supported this view in Italy and more recently it was advocated by Koch. Just as in connection with the influence of Koch’s great prestige much harm was done in prophylaxis against bovine tuberculosis so in this matter of quinine in the causing of blackwater the influence was unfortunate because many persons with severe malaria now refuse to take the specific quinine for fear of bringing on haemoglobinuria.