In November 1994 a major outbreak of acute gastroenteritis was experienced in the municipality of Klaebu (4,486 inhabitants). To investigate the course and extent of the epidemic, a questionnaire was mailed to all 1,573 households. The returned questionnaires covered 2,943 persons, of whom 1,640 (56%) were reported ill with the following most common symptoms: Nausea (83%), vomiting (78%), diarrhoea (72%) and abdominal pain (70%). The epidemic curve was typical of a common source epidemic. The incidence of vomiting turned out to have three distinct peaks with an interval of 26 hours, probably due to secondary infections. Two specimens that were examined by electron microscope showed typical Norwalk virus structures, and this virus was assumed to be the etiologic agent. The actual cause of this common-source epidemic was found to be inadequate chlorination. The onset of symptoms in different parts of the municipality showed that time taken for the infection to be transported from the reservoir to the consumers was longer than expected. Such delays allow for information on preventive measures to be communicated via the radio and other media.