OBJECTIVE: The incidence of inflammatory bowel disease (IBD) is increasing in Eastern Europe. The reasons for these changes remain unknown. The aim of this study was to investigate whether an East-West gradient in the incidence of IBD in Europe exists. DESIGN: A prospective, uniformly diagnosed, population based inception cohort of IBD patients in 31 centres from 14 Western and eight Eastern European countries covering a total background population of approximately 10.1 million people was created. One-third of the centres had previous experience with inception cohorts. Patients were entered into a low cost, web based epidemiological database, making participation possible regardless of socioeconomic status and prior experience. RESULTS: 1515 patients aged 15 years or older were included, of whom 535 (35%) were diagnosed with Crohn's disease (CD), 813 (54%) with ulcerative colitis (UC) and 167 (11%) with IBD unclassified (IBDU). The overall incidence rate ratios in all Western European centres were 1.9 (95% CI 1.5 to 2.4) for CD and 2.1 (95% CI 1.8 to 2.6) for UC compared with Eastern European centres. The median crude annual incidence rates per 100,000 in 2010 for CD were 6.5 (range 0-10.7) in Western European centres and 3.1 (range 0.4-11.5) in Eastern European centres, for UC 10.8 (range 2.9-31.5) and 4.1 (range 2.4-10.3), respectively, and for IBDU 1.9 (range 0-39.4) and 0 (range 0-1.2), respectively. In Western Europe, 92% of CD, 78% of UC and 74% of IBDU patients had a colonoscopy performed as the diagnostic procedure compared with 90%, 100% and 96%, respectively, in Eastern Europe. 8% of CD and 1% of UC patients in both regions underwent surgery within the first 3 months of the onset of disease. 7% of CD patients and 3% of UC patients from Western Europe received biological treatment as rescue therapy. Of all European CD patients, 20% received only 5-aminosalicylates as induction therapy. CONCLUSIONS: An East-West gradient in IBD incidence exists in Europe. Among this inception cohort--including indolent and aggressive cases--international guidelines for diagnosis and initial treatment are not being followed uniformly by physicians.