In a recent article in The Journal of the American Medical Association, Lichtenberg, Werner and Lueck1 questioned both the pathogenicity of the fusospirochetal organisms and the success of treatment with the arsenicals. After a number of years of experience with the disease in the laboratory and in the clinic, I have arrived at the conclusion diametrically opposed to that of Lichtenberg and his associates, that this group of anaerobic organisms has under certain conditions a primary pathogenicity, and that arsenical therapy is usually the most efficient treatment. In view of this difference of opinion, it seems advisable to review the evidence in favor of the pathogenicity of this group of organisms and to report some cases which illustrate the indications, dangers and limitations of arsenical therapy in fusospirochetal infections.In 1894, Veillon2 observed in necrotic, foul lesions of the throat the following mixture of organisms: cocci, Leptotriches vibriones, Spirochaeta denticola and fusiform bacilli. In December of the same year, Plaut3 described Spirochaeta dentium and Miller's bacillus in pseudodiphtheritic angina. Following Vincent's4 observations in 1896 and 1899, this type of infection was accepted as a clinical entity. The gradual development of knowledge of the disease has been reviewed in a recent monograph.5