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BACKGROUND: It has been suggested that the incidence of inflammatory bowel disease (IBD), which includes ulcerative colitis (UC) and Crohn's disease (CD), is three or more times higher in northern than in southern Europe. The aim of this EC funded study was to investigate this apparent variation by ascertaining the incidence of IBD across Europe. METHODS: For the period 1 October 1991 to 30 September 1993 all new patients diagnosed with IBD were prospectively identified in 20 European centres according to a standard protocol for case ascertainment and definition. FINDINGS: Altogether 2201 patients aged 15 years or more were identified, of whom 1379 were diagnosed as UC (including proctitis), 706 as CD, and 116 as indeterminate. The overall incidence per 100,000 at ages 15-64 years (standardised for age and sex) of UC was 10.4 (95% confidence interval (95% CI) 7.6 to 13.1) and that of CD was 5.6 (95% CI 2.8 to 8.3). Rates of UC in northern centres were 40% higher than those in the south (rate ratio (RR) = 1.4 (95% CI 1.2 to 1.5)) and for CD they were 80% higher (RR = 1.8 (95% CI 1.5 to 2.1)). For UC the highest reported incidence was in Iceland (24.5, 95% CI 17.4 to 31.5) and for CD, Maastricht (The Netherlands; 9.2, 95% CI 6.5 to 11.8) and Amiens (north west France; 9.2, 95% CI 6.3 to 12.2). The lowest incidence of UC was in Almada (southern Portugal) (1.6, 95% CI 0.0 to 3.2) and of CD in Ioannina (north west Greece) (0.9, 95% CI 0.0 to 2.2). An unexpected finding was a difference in the age specific incidence of UC in men and women with the incidence in women but not men declining with age. INTERPRETATION: The higher overall incidence rates in northern centres did not seem to be explained by differences in tobacco consumption or education. Nevertheless, the magnitude of the observed excess for both conditions is less than expected on the basis of previous studies. This may reflect recent increases in the incidence of IBD in southern Europe whereas those in the north may have stabilised.