An outbreak of lymphocytic choriomeningitis (LCM) associated with pet hamsters adquired through nationwide distributor was recognized in New York in February 1974. In upstate a New York, 57 cases occurred in a four-month period. Complete information was obtained on 49 patients ranging in age from 3 to 70 years. Characteristic findings included fever (90%), headache (85%), and severe myalgia (80%). Typically, laboratory findings consisted of a normal blood cell count and cerebrospinal fluid with mononuclear leukocyte pleocytosis and moderate hypoglycorrhacia. Al patients had contact with hamsters from one distributor. Most of the hamsters were acquired in mid to late December 1973, but some as late as March 1974. From each of eight families in which hamsters were available, at least one hamster had detectable complement-fixing antibodies for LCM. Infection rates within families varied with location and type of hamster cage; open cages and cages situated in common living areas were associated with highest infection rates (45% and 52%, respectively). Severity of illness was not associated with direct contact with hamsters. Onset of illness occurred between 8 and 90 days after initial exposure to hamsters.