Atrial fibrillation is an irregular and often rapid heart rate. Although atrial fibrillation itself usually isn't life-threatening, it is a serious medical condition that sometimes requires emergency treatment. Atrial fibrillation can lead to blood clots. If a blood clot forms, it could dislodge from the heart and travel to the brain, block blood flow to the brain and cause a stroke. The risk of developing a stroke is about 5 times higher in patients with atrial fibrillation than in patients without atrial fibrillation. Life-long use of blood thinners, like warfarin, is recommended to reduce the formation of blood clots and to prevent strokes. However, it is known that approximately 50% of patients with atrial fibrillation stop its use within one year. People who discontinue their blood thinners carry an increased risk of stroke. However, the actual level of risk after discontinuation of blood thinners is unclear. To better understand the consequences of stopping use of blood thinners we will undertake a study in patients with newly diagnosed atrial fibrillation who started treatment with blood thinners. We will compare the risk of stroke among those who then discontinued use of blood thinners with those who continued use of blood thinners.