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Several retrospective studies have reported seasonal variations in the relapse of ulcerative colitis, and two studies have found seasonality in the onset of ulcerative colitis, with a peak from August to January. This study was designed to investigate possible seasonal variations of onset of ulcerative colitis (UC) and Crohn's disease (CD). Patients with symptoms of one year or less were recruited from a prospective study of the incidence of inflammatory bowel disease, and the onset of symptoms was recorded month by month for four consecutive years. A total of 420 patients with UC and 142 patients with CD were included. There was monthly seasonality (p = 0.028) in symptomatic onset in December and January for UC but not for CD. It was found that environmental agents with known seasonality can be of importance for the seasonal variations of disease onset in UC.