Background Conversion of atrial fibrillation (AFib) and flutter (AFlt) to sinus rhythm results in a transient mechanical dysfunction of atria (atrial stunning). Methods used as a means of assessing atrial stunning, atrial stunning after conversion of atrial fibrillation/flutter, and the cause, mechanisms, determinants of the extent, and drugs affecting atrial stunning were examined. Methods Studies on the subject, identified through a comprehensive literature search, were thoroughly evaluated. Results and Conclusions Left atrial (LA) stunning has been reported with all modes of conversion of AFib/AFlt to sinus rhythm. The incidence of LA stunning is 38% to 80%. Spontaneous echocardiographic contrast, LA appendage (LAA) flow velocities and emptying fraction, transmitral inflow velocity of atrial wave (A-wave), time-velocity integral of A-wave, and atrial filling fraction have been used as means of assessing LA stunning. The data on right atrial (RA) stunning are limited, but parallel findings have been reported in the right atrium. Atrial stunning does not develop after the unsuccessful attempts of cardioversion or on delivery of electric current to the heart without AFib/AFlt, and it is a function of the underlying AFib/AFlt manifesting at the restoration of sinus rhythm. Tachycardia-induced atrial myopathy and chronic atrial hibernation are suggested mechanisms. Duration of preceding AFib/AFlt, atrial size, and underlying heart disease are determinants of the extent of atrial stunning. Verapamil, dofetilide, and acetylstrophenathidine have been shown to attenuate or protect from atrial stunning in animal or small human studies. A comprehensive knowledge of atrial stunning would be helpful in selecting the patients for, and the duration of, anticoagulation therapy after cardioversion. (Am Heart J 2002;144:11-22.)