Sleep apnea (SA) is more prevalent in patients with atrial fibrillation (AF), but the impact of cardioversion on disordered breathing is unknown. Thus, we investigated the influence of restoring sinus rhythm in patients with AF and atrial flutter (AFlut) on SA. The 16 patients (mean age 63.1 +/- 11.2) with AF (n = 6) or AFlut (n = 10) and SA (apnea-hypopnea index >10) received cardioversion or ablation of cavotricuspid isthmus. We compared the severity of SA by sleep polygraphy under AF/Aflut with the first night after restoring sinus rhythm and after 4 weeks. Apnea-hypopnea index before and immediately after restoring sinus rhythm was similar (31.7 +/- 13.2 vs 30.1 +/- 15.7, p = NS) despite a significantly reduced heart rate (86.7 +/- 26.5 vs 67.8 +/- 11.9 beats/min, p <0.02). After 4 weeks, apnea-hypopnea index remained unchanged (38.1 +/- 18.1, p = NS) although heart rate was further reduced (61.8 +/- 8.8 beats/min, p <0.003). In our study, SA could not be improved by cardioversion of AF/AFlut. Therefore, although it is well known that SA leads to AF, eliminating AF does not cure or improve SA. In conclusion, our study shows that AF should be regarded more as an innocent bystander than a causative or aggravating condition in SA.