Multifocal atrial tachycardia, also designated "chaotic atrial tachycardia," was identified in the records of 31 patients. It was particularly noted that the arrhythmia progressed to atrial fibrillation or flutter in 17 cases (55%). Unifocal or multifocal premature atrial contractions preceded the arrhythmia in 20 cases (64%). The arrhythmia developed during an acute illness in 18 cases (58%). However, it occasionally occurred in paroxysms without an apparent cause in patients with chronic disease. Significant acute or chronic pulmonary disease was present in 12 cases (39%). The arrhythmia was not associated with digitalis toxicity or with rhythm disturbances known to occur in digitalis intoxication such as paroxysmal atrial tachycardia with block. In general, digitalis therapy seemed to have little effect on the course of the arrhythmia, but in some cases it appeared to be beneficial, especially if atrial fibrillation supervened. In several patients there seemed to be a transition from multifocal premature atrial contractions through chaotic atrial tachycardia to atrial fibrillation. These observations suggested that chaotic atrial tachycardia might be a forewarning of atrial fibrillation and that the two arrhythmias may have a similar mechanism.