An epidemiological survey was undertaken to establish the importance of rotavirus as a cause of admission of children to hospital in Buenos Aires, Argentina. Between August 1978 and January 1980 rotavirus infection was detected in 62 (34%) of 180 infants and children admitted to hospital with acute gastro-enteritis. In identification of the virus, electron microscopy (EM) was found to be as sensitive as counterimmunoelectrophoresis (CIE). There was agreement between EM and CIE diagnosis in 90.5% cases. Adenovirus and coronavirus were detected by electron microscopy less often. A serological response to rotavirus was revealed by complement fixation test in 38 (72%) of 53 infected patients. Rotaviruses were detected throughout the year, but peak incidence occurred during the cooler months, May to August. Infection was commoner among children under six months of age. Diarrhoea was generally mild in these children and respiratory illness showed no significant association with rotavirus gastroenteritis. These results indicate that rotaviruses may be a major cause of infantile acute gastroenteritis in Buenos Aires, Argentina especially during the winter.