BACKGROUND: Environmental factors trigger the onset of inflammatory bowel disease (IBD) in genetically predisposed individuals. Exposure to seasonal external factors during the maturation of the immune system is suspected to be an inducing factor for IBD. Some studies suggested an association between the month of birth and the later development of IBD. We studied this putative relationship in a large cohort of Belgian patients with Crohn's disease (CD). METHODS: Data from 1025 patients born between 1935 and 1990 were collected. Diagnosis of CD was based on generally accepted clinical, endoscopic, and histologic criteria. As a control group, a cohort of 5125 non-IBD patients seen at the same hospital and matched for birth year and sex was used. Odds ratios were calculated using multivariate unconditional logistic regression including the matching variables and allowing for cyclic variation in risk with month of birth. RESULTS: A cyclic pattern described by a 4-month periodic function was observed with peaks in April and August. Moreover, being born in June significantly reduced the risk of developing CD later in life (P = 0.012). CONCLUSION: In this Belgian cohort, a significant association was found between the month of birth and later development of IBD; a significant reduced risk to develop CD was observed for people born in June. Moreover, environmental yearly reoccurring factors during pregnancy or postpartum might be associated with the occurrence of CD later in life.