I have to present the results of an inquiry into the possible importance of chronic bacterial infection as a factor in the causation of dementia præcox. My conclusions are based chiefly upon the study of thirty-two early cases in which it has been possible to make a sufficiently complete examination. I have also investigated by the same methods thirty cases of other forms of acquired insanity, and over 300 cases of nervous and other disorders in persons among the general population. I have to describe the chronic infections I have found in the cases of dementia præcox, and to endeavour to estimate their relation to the malady.