A clinical and virological study of an outbreak of porcine epidemic diarrhoea in a combined breeding and finishing pig herd used ELISA techniques to detect porcine epidemic diarrhoea virus in faeces and antibodies in blood. No seropositive pigs were found at the start of the outbreak. The first signs of the disease were observed in fattening pigs and the infection spread rapidly to pregnant sows, farrowing sows and their suckling pigs, gilts and weaners housed in separate barns. Depression and diarrhoea in the fattening pigs and pregnant sows were the clearest clinical signs. An endemic form of the disease developed which would not normally have been recognised as epidemic diarrhoea because no typical signs were apparent. Eleven groups of seronegative replacement gilts, which were brought in monthly, became infected with the virus and most of the groups developed a profuse diarrhoea lasting a few days. The infections and diarrhoea persisted in six- to 10-week-old pigs in separate barns. The suckling pigs and young weaners appeared to be spared from the infections.