An outbreak of severe gastroenteritis affecting 132 adults and children occurred in the small city of Mirassol, São Paulo, Brazil, in 1992. The outbreak of diarrhoeal disease had an abrupt onset and afflicted all age segments of the population. Group A rotavirus was the only pathogen associated with the epidemic. It was detected in 12 of the 27 (44%) stool specimens analyzed and was identified as serotype G2 rotavirus. Severe dehydration was common among adults and older children, and 35% of all the notified cases were hospitalized for parenteral rehydration. Contamination of the main water supply was the most likely source.