The occurrence of winter dysentery, diagnosed by farmers and defined as an outbreak of diarrhoea among at least 30 per cent of adult cattle in a herd, was monitored in 256 dairy herds in an area of central Sweden. The cumulative incidence of winter dysentery between April 1988 and March 1989 was 28.5 per cent. A typical outbreak lasted for one to two weeks and 74 per cent of the outbreaks occurred between November and January. A decrease in milk yield was reported in 90 per cent of the affected herds and the cows showed respiratory signs in 57 per cent of them. There was a significant (P < 0.05) association between the occurrence of fever and coughing. In 31 per cent of the outbreaks the farmer also noticed diarrhoea among the calves. One-third of the affected herds had experienced an outbreak within the previous four years and 18 per cent had at least one further outbreak during the following two years. There was a significantly (P < 0.05) lower disease score in herds that had had an outbreak within the previous four years than in herds which had had a less recent or no previous outbreak, indicating the development of temporary immunity to the causative agent.