SUMMARY Six epizootics of encephalomyelitis in suckling pigs in Minnesota were attributed to infection with hemagglutinating encephalomyelitis virus. The disease occurred in 74 litters of pigs and was characterized by sudden onset of tremors, inappetence, weakness, ataxia, and hyperesthesia, with high morbidity and case fatality rate. Pathologic changes consisted of marked nonsuppurative, nondemyelinating encephalomyelitis characterized by perivascular mononuclear cuffing, gliosis, neuronal death, and satellitosis. Clinical disease was limited principally to suckling pigs during a single farrowing period and did not recur in the herds involved during the ensuing 18 months.