Observations on psychoses associated with the influenza epidemic of 1918. The material is divided into two groups—(1) cases in which influenza was the direct cause of the psychosis, fever-delirium, and post-febrile amentia; (2) cases in which the toxic condition aroused a latent tendency to mental disorder. Naturally only the severest cases of delirium came under treatment in the clinic and fifteen are included in this study. The symptoms were confusion, terror, psycho-motor excitement, and delusions of persecution and poisoning associated with hallucinations. The majority of cases were men; the earliest onset was the second day of fever and the latest the eighth; the prognosis in respect to life was bad, 12 cases ending fatally, whilst the mental symptoms subsided with the fall of the temperature in the three cases which recovered. Post-febrile cases numbered 30, the greater portion of whom were women. The symptoms were association disturbances, confusion, and hallucinations, and the psychosis began with sleeplessness, fatigue, irritability and nocturnal hallucinations, some cases of short duration remaining at this stage. The prognosis was good, and no case ended fatally. The interval between the fall of temperature and the onset of mental symptoms varied from two to fourteen days. In no case was the influenza the sole factor in the production of the post-febrile psychoses. An hereditary factor could be excluded, but lactation, pregnancy, alcohol, or malnutrition appeared as subsidiary causes. In view of the relatively small number of psychoses in a widespread epidemic the writer concludes that there must be an unknown causal factor in these cases.