ObjectiveTo examine the relationship between Mediterranean diet and risk of later-onset Crohn’s disease (CD) or ulcerative colitis (UC).DesignWe conducted a prospective cohort study of 83 147 participants (age range: 45–79 years) enrolled in the Cohort of Swedish Men and Swedish Mammography Cohort. A validated food frequency questionnaire was used to calculate an adherence score to a modified Mediterranean diet (mMED) at baseline in 1997. Incident diagnoses of CD and UC were ascertained from the Swedish Patient Register. We used Cox proportional hazards modelling to calculate HRs and 95% CI.ResultsThrough December of 2017, we confirmed 164 incident cases of CD and 395 incident cases of UC with an average follow-up of 17 years. Higher mMED score was associated with a lower risk of CD (P_trend=0.03) but not UC (P_trend=0.61). Compared with participants in the lowest category of mMED score (0–2), there was a statistically significant lower risk of CD (HR=0.42, 95% CI 0.22 to 0.80) but not UC (HR=1.08, 95% CI 0.74 to 1.58). These associations were not modified by age, sex, education level, body mass index or smoking (all P_interaction >0.30). The prevalence of poor adherence to a Mediterranean diet (mMED score=0–2) was 27% in our cohorts, conferring a population attributable risk of 12% for later-onset CD.ConclusionIn two prospective studies, greater adherence to a Mediterranean diet was associated with a significantly lower risk of later-onset CD.