It has been a common practice to perform cardioversion of acute (<48 hour) AF without any anticoagulation. The new European guidelines recommend that anticoagulation should be started already before cardioversion in all patients with CHADS2VASC score indicating need for long-term oral anticoagulation. The evidence behind these guidelines is, however, scarce. In this retrospective study we collect data on the safety of acute AF cardioversion from emergency rooms of two university hospitals and one secondary referral center from the years 2003-2010.