The incidence of Crohn's disease increased sixfold from 1962 to 1987 in the county of Copenhagen. The mean annual incidence for 1979-87 was 4.1 per 10(5) inhabitants. The increase was found equally in both sexes, with an approximately 40% higher incidence in women. The maximal incidence was found in the 15- to 24-year age group, being 12.8 per 10(5) per year for women and 6.0 per 10(5) per year for men, as mean of the period 1979-87. The prevalence at the end of the study was 54 per 10(5) inhabitants, 46 per 10(5) in men and 63 per 10(5) in women. The clinical appearance of the disease at the time of diagnosis was remarkably similar during the study period with regard to both the localization of disease and the clinical symptoms and signs. A slightly higher percentage of the patients, however, were in high disease activity at diagnosis in the later years of the study.