The incidence of Crohn's disease was assessed in a population based study in Copenhagen County from 1962-1987. The incidence increased six fold during the study period from 0.62/10(5) in 1962 to a mean of 4.1/10(5) from 1979-1987, equally in both sexes. The highest incidence was found in the age group 15-24 years: 12.8/10(5) for women and 6.0/10(5) for men (as mean of the period 1979-1987). The prevalence at the end of the study was 54/10(5). The clinical symptoms and extent of disease at diagnosis did not change over the time, but a significantly higher disease activity was found in the 1980's, 74% (1962-1979) vs 84% (1980-1987), p = 0.04. The increase in incidence of Crohn's disease is well established in most parts of the industrialized world. The analysis shows that it is not due to a new disease entity, since the extent of disease, the age and sex distribution, and the symptoms remained unchanged during the 26 year study period.