A boy from New York traveling in Nova Scotia had olfactory hallucinations and other signs of temporal lobe involvement, leading to a diagnosis of herpes simplex encephalitis. The patient was treated with vidarabine and made a complete recovery. However, hemagglutination inhibition, complement fixation, and neutralization tests identified Powassan virus (POW) as the pathogen. Shortly before his trip to Nova Scotia, the patient had traveled in an area where POW encephalitis had occurred in humans (the eastern part of the state of New York), and he also came in contact with a known reservoir of POW infection (a groundhog) at home.