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Background. Cardiovascular events such as acute myocardial infarction and stroke vary seasonally, with a peak incidence during the winter and a trough during the summer. Few data exist on the seasonal variation of atrial fibrillation. Methods. We examined the association among season, outdoor temperature, and risk of atrial fibrillation. We identified 32,992 subjects, age 50-89 years, in a random sample of half the people with an incident hospital discharge diagnosis of atrial fibrillation in the Danish National Hospital Discharge Register from 1980 to 1993. We used a periodic regression model to evaluate any seasonal distribution. Results. We found a winter peak and a summer trough in the incidence of atrial fibrillation. The relative incidence of atrial fibrillation during winter as compared with summer, estimated as the ratio of the incidence in the month of the peak relative to the incidence in the month of the trough, was small (relative risk = 1.20; 95% confidence interval = 1.12-1.29). Stratification by sex, age, and the presence or absence of diabetes and underlying cardiovascular diseases did not affect the seasonal pattern. We also found an inverse relation between mean outdoor temperature and incidence of atrial fibrillation. Conclusion. The risk of atrial fibrillation was modestly higher during the winter and was inversely associated with outdoor temperature
