Spontaneous conversion of established atrial fibrillation to atrial flutter was observed 14 times, and conversion to paroxysmal auricular tachycardia (PAT) with atrioventricular block was observed four times within a five-year period. In several patients, atrial fibrillation apparently had been persistent for several years. All four episodes of PAT with block were due to digitalis toxicity. Digitalis intoxication could be implicated in only three episodes of atrial flutter, and no definitive etiology could be identified in the remainder. The change to atrial flutter often presented a therapeutic dilemma since these patients were usually receiving digitalis. Compounding the problem is that differentiation of PAT with block from atrial flutter is not always possible. This fact was exemplified by a patient whose serial tracings revealed a continuum from the former to the latter as digitalis toxicity abated.