An outbreak of nosocomial echovirus 11' illness occurred between July 22 and August 12, 1983, infecting 14 infants in the nurseries at a hospital. Stool, throat or cerebrospinal fluid specimens for viral isolation were obtained from 142 and serum pairs from 98 of the 192 infants exposed to the nurseries during the outbreak. Infection was determined by isolation of virus from stool or cerebrospinal fluid and/or a 4-fold rise in neutralizing antibody to echovirus 11'. Eight infants were severely ill with seizures, pleocytosis or apnea; one infant died. Four infants were mildly ill; two were asymptomatic. Immune serum globulin, administered to all hospitalized infants on July 31, did not appear to attenuate echovirus 11' illness. Infants present in the Intermediate Care Unit greater than 48 hours were more likely to become infected than infants present less than or equal to 48 hours (attack rates, 25 and 3%, respectively). Among infants present in the Intermediate Care Unit, illness was associated with gavage feeding, mouth care and being a twin (P less than 0.05). These findings support the importance of good hygienic practices in preventing nosocomial viral infections.