An epidemiological study of Crohn's disease has been conducted in northeastern Scotland since 1955 and in the Orkney and Shetland Islands since 1966 through the end of 1988. There were 856 verified cases on the mainland and 58 in the northern isles. There has been no change in the sex ratio on the mainland in recent years, with 63% of patients female. Younger men and women living in the city of Aberdeen are more liable to develop Crohn's disease than rural inhabitants; the reverse is true for men over 60 years of age. The annual incidence of new cases has increased; within the city of Aberdeen it was 11.6 per 100,000 during 1985-1987. The point prevalence on December 31, 1988, for northeastern Scotland was 147 per 100,000. Family histories of the disease were noted in 8.3% of cases. The disease now affects more distal parts of the intestinal tract. There is no clear correlation between these changes and variations in the environment or lifestyle of the population. In the Orkney Islands the overall incidence was 6.1 per 100,000, and there was a male preponderance. In the Shetland Islands the incidence was 5.7 per 100,000 during 1966-1988. A family history of Crohn's disease was more common in the northern islands than on the mainland.