The atria are the upper chambers of the heart. Atrial fibrillation and atrial flutter are abnormal heart rhythms that occur when the atria do not beat normally. Atrial fibrillation is the most common abnormal heart rhythm. It involves a rapid, irregular heartbeat that can cause symptoms such as pounding in the chest or lightheadedness. Atrial flutter is less common and involves a very rapid but regular heartbeat, a “short circuit” in the atria. Heart, lung, and thyroid disease increase the risk for atrial fibrillation and atrial flutter, but both can occur in healthy people. When they occur in people without underlying disease, these heart rhythms are called “lone atrial fibrillation” or “lone atrial flutter.” The most serious complication of these conditions is the formation of blood clots in the atria that travel to the brain, causing strokes or transient ischemic attacks. In transient ischemic attack, blockage of blood flow to the brain is temporary and no permanent damage occurs. In stroke, blockage of the blood flow lasts long enough that a section of brain tissue dies. Stroke occurs in about 10 of every 100 patients with atrial fibrillation each year. Treatment with blood thinners can reduce the risk for this complication and is standard treatment for most patients with atrial fibrillation. Because lone atrial flutter is uncommon, we know little about what happens to patients with this condition.