Conventional virologic investigations generally failed to implicate viruses as a cause of acute gastroenteritis in young children. When negative-contrast stain electron microscopy was used for the examination of stool samples from affected patients, several candidate etiologic agents were discovered. Rotavirus (otherwise known as orbivirus, reo-like agent, and infantile gastroenteritis virus) emerged as an important causative agent. In a considerable number of patients, however, no causative agent was identified. Several other candidate etiologic viruses now "stand in the wings" awaiting additional evidence as to their role in the pathogenesis of acute gastroenteritis. In a study period of 12 months, 669 patients were found to be shedding virus in association with symptoms of diarrhea, vomiting, and temperature elevation. Approximately one third of these patients acquired their infection in hospital.