Arthropod-borne virus encephalitis in the U.S.A. includes LaCrosse, St Louis, western equine, eastern equine, Venezuelan equine, and Powassan in that order of frequency. Diagnosis can be aided by the history of seasonal occurrence, climate, geographic location, exposure to vectors, and age of the patient. The definitive diagnosis is usually made by serological tests such as neutralization, complement-fixation, hemagglutination-inhibition, and immunofluorescence, the radioimmune assay and the enzyme-linked immunosorbent assay show promise of future utility. These diseases are prevented by vector control. It is unlikely that vaccines or anti-viral agents will have application in the near future.