Objective: Previous research has yielded conflicting data as to whether the natural history of inflammatory bowel disease follows a seasonal pattern. The purpose of this study was to determine whether relapse of inflammatory bowel disease follows a seasonal pattern either across a cohort of patients or within individual patients.

Methods: We used 1988 to 1997 data from the General Practice Research Database to conduct a retrospective cohort study of 1587 patients with Crohn's disease (mean age at start of follow-up, 41 +/- 17 years) and 2773 patients with ulcerative colitis (mean age at start of follow-up, 48 +/- 16 years). Flares of disease were identified by receipt of a new prescription for either corticosteroids or 5-ASA medications following an interval of at least 4 months without prescriptions for either class of medication. Logistic regression was used to adjust the association of season of the year and flare of disease for potential confounding variables.

Results: There was no association between season of the year and flare of Crohn's disease (P = 0.66). Season of the year was only weakly associated with flares of ulcerative colitis (P = 0.02). Compared with winter, spring had very slightly higher rates of flares (OR = 1.13, 95% CI: 1.05-1.23). We did not observe seasonal patterns within individual patients experiencing multiple flares (P > 0.05 for Crohn's disease and ulcerative colitis).

Conclusions: Although we observed a slight increase in exacerbations of ulcerative colitis in the spring, in general, these data do not support an association between season of the year and flares.