An epidemiologic study of Crohn's disease, comprising the population of the two most northerly counties of Sweden and covering the 8-year period 1974-1981, was carried out. The basic population was about 510,000, and the area is regarded as rural, with a mean density of 3 inhabitants per km2. In all, 199 patients with a hospital discharge diagnosis of Crohn's disease were identified. A mean annual incidence of 4.9 per 10(5) inhabitants and a peak incidence of 6.7 were found. There was no sex difference and no obvious change in incidence during the time period studied. The highest incidence was observed in young adults, and ileal disease predominated. A significantly higher incidence was observed in the town of Umeå than in the rest of northern Sweden. The increase was confined to the ages between 20 and 40 years. The present study concludes that the incidence of Crohn's disease is high in northern Sweden, even though it is a sparsely populated rural area. The incidence figures are similar to those obtained for other parts of Sweden.