What have we learnt about the role of the environment and natural course of IBD in the new millennium? 20-year follow-up of the IBSEN cohort.

The incidence and prevalence of IBD, both Crohn's disease (CD) and ulcerative colitis (UC), have increased in recent years, especially in industrialized countries. Still, the etiology of IBD remains largely unknown. Most research on IBD before the 1990s was conducted on selected patient populations. Selected patient populations are likely to introduce important bias and limit the interpretation and generalizability. The inclusion of both incident and prevalent cases or the inclusion of incident cases over long periods of time (decades) might also introduce bias due to changes in treatment regimens and socioeconomic factors over timer (time-trend bias). Consequently, the choice of a well-characterized population-based inception cohort provides the best opportunity to describe the natural course of a disease. The IBSEN (Inflammatory Bowel Disease in South-Eastern Norway) study followed a large population-based cohort of newly diagnosed IBD patients for 20 years and has contributed significantly to the knowledge of the natural course of IBD.