The diagnosis of encephalitis, particularly of certain types of encephalitides, has been frequently described as a "wastebasket clinical designation."1It is the aim of this study to contribute to available knowledge by a follow-up investigation of all the cases in which a diagnosis of encephalitis or encephalomyelitis was made clinically or at autopsy at the Boston City Hospital from 1928 to and including 1938. MATERIAL The group examined consisted of 100 patients. The total hospital admissions during the same period were 368,000. Patients suffering from postencephalitic parkinsonism, Sydenham's chorea and syphilis of the central nervous system were not included in the group. Of the 100 patients studied, 45 were in the Neurological Unit, while the remaining 55 were distributed in the various services of the Boston City Hospital. Thirty-seven died in the hospital; for 19 of these complete autopsy reports and microscopic slides were available. Thirty-six of the remaining