Viral illnesses in Strong Memorial Hospital were monitored over a 17-month period. Using criteria based primarily on the incubation periods for a number of common virus infections, the infections we found were classified as hospital- or community-acquired. Hospital-acquired viral infections occurred on most hospital services; the majority of infections occurred on the pediatric and psychiatric services. Infections due to herpesviruses were seen more frequently in a group of patients aged 14 years or older, while infections in patients aged three years or younger were more likely to be due to respiratory syncytial virus, influenzavirus, adenovirus, or parainfluenza virus. Patients with nosocomial infections due to viruses were hospitalized an average of 9.3 days longer than uninfected controls; thus nosocomial viral infections result in increased costs of hospitalization.