1. We have observed 331 delirious episodes occurring in the course of 1324 hyperpyrexia sessions administered to 200 patients suffering from various pathological conditions. There were nine severe or classical deliria.2. Attempts were made to correlate the occurrence of the deliria with the disease, the height and duration of the fever, and the administration of sedatives.3. In case studies specific factors were recorded which influenced the development and severity of delirious episodes.4. We noted individual features in patients with recurrent deliria which reflected their dominant interests, habit patterns, and other personal characteristics.5. The prevention and treatment of delirious episodes are discussed.Abstract Teaser