Seasonal variation in exacerbations of ulcerative colitis.

The monthly distribution of relapses in ulcerative colitis was analysed retrospectively to assess seasonal variation. In 338 patients who were regularly followed up at the colitis clinic, 1013 relapses occurred during the period 1977 to 1988. A peak of relapses occurred in the spring and autumn, with a decrease in the summer; the ratio of the observed to expected number of relapses was 201:255.3 for the summer, 289:252.6 for the autumn, 243:249.8 for the winter, and 280:255.3 for the spring. In contrast, month of diagnosis was evenly distributed without seasonality. The data support the concept that exacerbations in ulcerative colitis are influenced by seasonal factors.