During the 1995 river floods in eastern Norway, 7,000 people were forced to abandon their houses and the public water supplies of some 150,000 people were threatened. The National Institute of Public Health feared outbreaks of waterborne diseases. We supplemented the local preventive efforts with expert advice and public information. We emphasised measures to maintain safe water supplies and to provide information on safe management of flood water during evacuation and clean-up. We observed no increase in the incidence of acute gastroenteritis or other possibly flood-related communicable diseases among the 329,000 people living in the municipalities affected by the floods. We conclude that the floods did not cause a measurable increase in the incidence of communicable diseases. This was probably due to some extent to the measures taken to protect the water supplies.