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Natural history studies provide invaluable data on the disease course. Rungoe et al1 report changes in medical treatment and surgery rates over the past three decades (1979–2011) using a large Danish population-based cohort of 48 467 patients with IBD issued from a Danish health administrative database. Interestingly, the authors found a decrease in surgery rates in both Crohn's disease (CD) and UC. Changes in surgery rates were accompanied by a significant decrease in the use of five aminosalicylic acids (5-ASA) and corticosteroids. The use of 5-ASA significantly decreased from 51% in 1995–2002 to 19% in 2003–2011 in CD; surprisingly, it also decreased in UC from 63% to 51%, while in UC 5-ASA remain the backbone therapy for mild-to-moderate UC according to recent international guidelines.2 As expected, there was a dramatic increase between 1995–2002 and 2003–2011 in the use of thiopurines and anti-tumor necrosis factor (TNF) 10 years after diagnosis: from 32% to 39% for azathioprine and from 10% to 23% for anti-TNF in CD. In UC, this increase was smaller than in CD and did not reach statistical significance. These rates are much lower than those reported in a French referral centre where the cumulative probabilities of receiving thiopurines and anti-TNF 5 years after CD diagnosis were 70% and 65%, respectively.3 Such discrepancy between referral centres and population-based studies demonstrates the importance to conduct cohort studies at the population level to avoid selection bias from referral centres in which patients with more severe disease are usually treated.