Liao JN, et al. - The risk of ischemic stroke was compared across different seasons to test if cold weather increases the risk of stroke in atrial fibrillation (AF). Using the National Health Insurance Research Database in Taiwan, researchers enrolled 289,559 AF patients from 2000 to 2012; after a mean follow-up of 3 years, 34,991 experienced ischemic stroke. Observations revealed a seasonal variation of ischemic stroke incidence in AF patients. An increased risk of stroke was observed on days with an average temperature < 20^oC, suggesting an interaction of environmental factors with AF-related stroke.