The epidemiology of bovine spongiform encephalopathy (BSE) in Northern Ireland from 1988, when it was first confirmed, to the end of 1995 is described. All cases of BSE were subjected to a detailed epidemiological investigation, complemented by data from the national animal health records on every bovine animal. Data are presented on 1680 cases. Many of the epidemiological features of the disease were similar to those reported in Great Britain, but the incidence in Northern Ireland was approximately one-tenth that in Great Britain. The epidemic increased to a peak of 56 cases per month in January 1994, and decreased to nine cases in December 1995. Statutory intervention banning the use of meat and bone meal in ruminant feed in January 1989 has produced a marked and continuing reduction in the incidence. The majority of the cases were in Northern Ireland cattle, but 83 cases were imported from Great Britain and five from the Republic of Ireland. Many of the key epidemiological features have remained constant throughout the epidemic: the greater incidence of BSE in dairy herds than in beef suckler herds, the low within-herd incidence, the variation in incidence with herd size, the breed distribution, the distribution of the reported clinical signs and the proportion of purchased cases. Although the source of the BSE epidemic in Northern Ireland has not been established conclusively, the evidence suggests that the importation of meat and bone meal and protein concentrates from Great Britain may have been responsible.