Laboratory evidence of recent or current lymphocytic choriomeningitis (LCM) virus infection was obtained in 60 patients. Twelve had diagnosis of central nervous system (CNS) infection: four of meningoencephalitis and eight of meningitis. Thirty-four patients had a grippe-like syndrome. Fifty-nine had had contact with pet hamsters. All of the 24 patients whose pets were studied had been exposed to one or more hamsters with serologic evidence of past LCM virus infection. The data implicate pet hamsters as a source of LCM in man. A continuous effective control of LCM virus in pet hamsters appears impractical. At present, the only feasible way to prevent further cases is the physician's special attention to the possibility of rodent contacts of patients with CNS disease and early laboratory confirmation of suspected cases of human LCM virus infections.