Background: Smoking increases CD risk. The aim was to determine if smoking cessation at, prior to, or following, CD diagnosis affects medication use, disease phenotypic progression and/or surgery. Methods: Data on CD patients with disease for ≥. 5. yrs were collected retrospectively including the Montreal classification, smoking history, CD-related abdominal surgeries, family history, medication use and disease behaviour at diagnosis and the time when the disease behaviour changed. Results: 1115 patients were included across six sites (mean follow-up-16.6. yrs). More non-smokers were male (p. = 0.047) with A1 (p. <. 0.0001), L4 (p. = 0.028) and perianal (p. = 0.03) disease. Non-smokers more frequently received anti-TNF agents (p. = 0.049). (p. = 0.017: OR 2.5 95%CI 1.18-5.16) and those who ceased smoking prior to diagnosis (p. = 0.045: OR 2.3 95%CI 1.02-5.21) progressed to complicated (B2/B3) disease as compared to those quitting at diagnosis. Patients with uncomplicated terminal ileal disease at diagnosis more frequently developed B2/B3 disease than isolated colonic CD (p. <. 0.0001). B2/B3 disease was more frequent with perianal disease (p. <. 0.0001) and if i.v. steroids (p. = 0.004) or immunosuppressants (p. <. 0.0001) were used. 49.3% (558/1115) of patients required at least one intestinal surgery. More smokers had a 2nd surgical resection than patients who quit at, or before, the 1st resection and non-smokers (p. = 0.044: HR. = 1.39 95%CI 1.01-1.91). Patients smoking >. 3. cigarettes/day had an increased risk of developing B2/B3 disease (p. = 0.012: OR 3.8 95%CI 1.27-11.17). Conclusion: Progression to B2/B3 disease and surgery is reduced by smoking cessation. All CD patients regardless of when they were diagnosed, or how many surgeries, should be strongly encouraged to cease smoking.