Infection with Pneumocystis cariniiwas demonstrated immunohistochemically in the lungs of pigs 15 to 75 days of age from a herd with epidemic pneumonia due to the organism. The distribution of the organism was centered on the airways, and extended progressively with age from the alveolar ducts to the alveoli. In a retrospective immunohistochemical study of 245 newborn to adult pigs which were necropsied between 1988 and 1995, P carinii infection was found in 87 pigs (35-5 per cent) aged between 17 days to seven months. In the pigs aged between one and three months the infection rate was 63-1 per cent. Pigs from herds in which suckler and weaner pigs shared the same air space were more heavily infected than those from the herds in which they were reared separately. There were no regional or seasonal variations in the level of infection, and the infection was not associated with any single disease.