An atrial defibrillator applies cardioverting electrical energy to the atria of a human heart in need of cardioversion. The defibrillator includes an electrode pair and a sense amplifier associated with the atria of the heart for sensing electrical activity of the heart during a plurality of cardiac cycles to provide a cardiac signal. A detector is responsive to the cardiac signal for detecting cardiac events. During each cardiac cycle of the plurality of cardiac cycles, a time for counting is established wherein each time for counting has a total duration less than the duration of its corresponding cardiac cycle. A counter counts the cardiac events detected by the detector during the time for counting of the plurality of cardiac cycles to provide a cardiac event count. A comparator compares the cardiac event count to a predetermined cardiac event count. If the cardiac event count is greater than the predetermined cardiac event count, the atria are deemed to be in fibrillation and a cardiovertor applies cardioverting electrical energy to the atria to cardiovert the detected atrial fibrillation.