Objectives: To assess the success of electrical cardioversion for the treatment of new-onset atrial fibrillation in critically ill patients and to evaluate the stability of sinus rhythm in responders during the subsequent 24 hours. Design: Retrospective study. Setting: Twelve-bed cardiosurgical ICU at a university hospital. Patients: Seventy-two consecutive patients with postoperative new-onset atrial fibrillation (< 7 d of duration) treated by electrical cardioversion. Interventions: Electrical cardioversion using synchronized bipha-sic shocks. Measurements and Main Results: During 144 electrical cardio-versions, 209 shocks were delivered to 72 patients. Maximal energy (200 J) was used in 85% of shocks. Electrical cardiover-sion immediately restored sinus rhythm in 102 sessions (71%). Pretreatment with amiodarone did not increase the success rates. During the follow-up, the percentages of sinus rhythm decreased from 43% after 1 hour to 23% after 24 hours. However, at ICU discharge, 54 patients (75%) were in sinus rhythm. Of the 54 patients in sinus rhythm, only 18 (33%) converted to sinus rhythm after repeated cardioversions, whereas the remaining 36 (66%) did so spontaneously or with amiodarone. Conclusions: Biphasic electrical cardioversion in cardiosurgi-cal ICU patients was immediately successful in restoring sinus rhythm in 71% of sessions. However, early relapse of atrial fibril-lation was common in the 24-hour follow-up. At ICU discharge, the majority of patients were in sinus rhythm, but the efficacy of repetitive electrical cardioversion in restoring sinus rhythm was disappointing.