OBJECTIVES: We assessed the association of smoking at diagnosis of inflammatory bowel disease (IBD) on the need for an intestinal resection. METHODS: The Health Improvement Network was used to identify an inception cohort of Crohn s disease (n=1519) and ulcerative colitis (n=3600) patients from 1999 2009. Poisson regression explored temporal trends for the proportion of newly diagnosed IBD patients who never smoked before their diagnosis and the risk of surgery within 3 years of diagnosis. Cox proportional hazard models assessed the association between smoking and surgery, and effect modification was explored for age at diagnosis. RESULTS: The rate of never smokers increased by 3% per year for newly diagnosed Crohn s disease patients (incidence rate ratio (IRR) 1.03; 95% confidence interval (CI): 1.02 1.05), but not for ulcerative colitis. The rate of surgery decreased among Crohn s disease patients aged 17 40 years (IRR 0.96; 95% CI: 0.93 0.98), but not for ulcerative colitis. Smoking at diagnosis increased the risk of surgery for Crohn s disease patients diagnosed after the age of 40 (hazard ratio (HR) 2.99; 95% CI: 1.52 5.92), but not for those diagnosed before age 40. Ulcerative colitis patients diagnosed between the ages of 17 and 40 years and who quit smoking before their diagnosis were more likely to undergo a colectomy (ex-smoker vs. never smoker: HR 1.66; 95% CI: 1.04 2.66). The age-specific findings were consistent across sensitivity analyses for Crohn s disease, but not ulcerative colitis. CONCLUSIONS: In this study, the association of smoking and surgical resection was dependent on the age at diagnosis of IBD.