Atrial fibrillation and atrial flutter share a common reentrant mechanism. However, the relationship between these arrhythmias has not been systemically studied to date. To evaluate the degree to which these arrhythmias may alternate, consecutive Holter monitor recordings which showed fibrillation or flutter in 96 patients were reviewed. One half of the patients were studied after open-heart surgery and the other half for varying indications. One quarter of the patients had atrial flutter in addition to fibrillation, and this alternation with flutter was significantly associated with the use of a type 1A antiarrhythmic drug (p = 0.007), but not with the use of digoxin or beta blockers (p = NS for both). Furthermore, this alternation with flutter was more common in the postoperative group (p = 0.01). A history of embolization was less common in patients who were in the postoperative group (p = 0.003) and patients who had flutter in addition to fibrillation (p = 0.05).