A prospective epidemiological study of Crohn's disease in Malmö, Sweden, was carried out during the years 1958 to 1973. The mean annual incidence rate for the disease was 4.8 per 10(5) inhabitants at risk. The recorded incidence increased from 3.5 per 10(5) inhabitants during the initial 8-year period, to 6.0 per 10(5) inhabitants during the last 8 years of the study. The maximum annual incidence rate, 8.3 per 10(5) inhabitants in 1971, is the highest hitherto on record. A prevalence rate of 75.2 per 10(5) inhabitants on December 31, 1973 was recorded. The death rate was low: 0.15 per 10(5) inhabitants per year. The alidity of the observed high incidence rates and the rising secular trends are discussed.