Background: The impact of smoking at diagnosis and subsequent smoking cessation on clinical outcomes in Crohn's disease (CD)has not been evaluated in a population based cohort. Methods: Using a nationally representative clinical research database, we identified incident cases of CD between 2005 and 2014. We compared the following outcomes: overall corticosteroid (CS) use; flares requiring CS; CS-dependency and intestinal surgery between smokers and non-smokers at time of CD diagnosis. Differences in these outcomes were also compared between persistent smokers and smokers who quit within 2 years of diagnosis. Results: We identified 3553 patients with a new CD diagnosis over the study period of whom 1121 (32%) were smokers. Smokers at CD diagnosis had significantly higher CS-use (56% vs. 47%, p<0.0001), proportionally more CS-flares (>1 CS flare/yr: 9% vs 6%, p<0.0001), and higher CSdependency (27% vs. 21%, p<0.0001) than non-smokers. Regression analysis identified smoking at diagnosis to be associated with a higher risk of intestinal surgery (HR 1.64, 95%CI 1.16-2.52). There was a significantly higher proportion of 'quitters' who remained steroid-free through follow-up in comparison to 'persistent smokers' (45.4% vs. 37.5% respectively, p=0.02). 'Quitters' also had lower rates of CS-dependency compared to 'persistent smokers' (24% vs 33%, p=0.008). Conclusions: Smokers at CD diagnosis have higher CS-use, CS-dependency and higher risk of intestinal surgery. Quitting smoking appears to have beneficial effects on disease related outcomes including reducing CS-dependency highlighting the importance of offering early smoking cessation support. Study highlights 1) What is current knowledge: smoking is common amongst patients with Crohn's disease (CD) smoking may worsen outcomes in CD but the exact association remains unclear, and much of the available data comes from secondary and tertiary care centres there is only limited data on the impact of smoking cessation in outcomes in CD 2) What is new here: patients with CD who are smokers at the time of diagnosis have higher overall corticosteroid (CS) use compared to non-smokers CS dependency, as defined by international guidelines, is more frequent amongst CD patients who smoke at diagnosis patients with CD who smoke at diagnosis are two thirds more likely to undergo intestinal surgery patients with CD who stop smoking within the first two years of diagnosis have lower rates of CS dependency and are more likely to be steroid-free