Abstract So far, this paper has been confined largely to a discussion of the exciting agents of arthritis. Before closing, perhaps something should be said about the various predisposing influences which are at work in these diseases. In rheumatic fever the most important predisposing factor is environment. Rheumatic fever is a rare disease among the well-to-do. It is extremely common among the children of the poor. This increase of susceptibility in the poor is probably a combination of improper diet and exposure to cold. The well-to-do are better fed and better clothed and escape infection. Probably another predisposing factor is exposure to a carrier of the rheumatic fever virus. St. Lawrence 13 has pointed out the prevalence of rheumatic fever in two or more children of the same family. This may be heredity, but contagion may also be an important factor. In both rheumatic fever and infectious arthritis, focal infection is of vital significance. Statistics show that rheumatic fever cannot be prevented by the removal of all foci of infection, but the incidence is considerably reduced. Other factors are influential in the etiology of infectious arthritis. Among them, nervous or emotional shock should be stressed. It is surprising what a large percentage of these patients give a history of overwork or some emotional disturbance just prior to the onset of arthritis. Other factors, such as trauma and exposure to cold and dampness, sometimes provoke the first attack. I have seen a number of butchers who claim to have contracted arthritis from too many trips to the refrigerator. In hypertrophic arthritis, overweight and advancing years are the two great predisposing agents. Heredity also plays an important part. Nearly every patient with Heberden's nodes will give a history of a mother or father who was afflicted with the same condition. This discussion has been nothing more than a cursory review of a complex subject, but perhaps enough has been said to indicate that some of the mysteries of arthritis are on the way to solution, and that with a few more years' intensive research along bacteriologic and physiologic lines, the medical profession will have a fairly concise idea as to the true nature of rheumatism and arthritis.