 Heart failure and stroke are the two major complication of atrial fibrillation. Current treatment for atrial fibrillation is so focused to stroke prevention, but the risk assessment for heart failure is less highlighted. The most of patients with atrial fibrillation have cardiac functional limitation of variable degree. We hypothesize that the exercise test would reveal the subclinical cardiac dysfunction, and might be helpful to classify the patients with atrial fibrillation according to their exercise capacity real cardiac function. 