Background The Cox maze procedures have been shown to be safe methods for restoring sinus rhythm (SR) in patient with atrial fibrillation and often performed concomitant to mitral valve (MV) surgery. Cryothermy (Cryo) and Radiofrequency (RF) are available techniques to achieve atrial linear scars. The differences between the effect of these energy sources on late left atrial (LA) reverse remodeling are not fully described. Purpose This study aims to compare the impact of Cryo and RF procedures on left atrial (LA) size and function one year after Cox-maze IV concomitant to MV surgery using 3-dimensional echocardiography (3DE). Methods Seventy-two patients with MV disease and atrial fibrillation were randomized to either Cryo (35) or RF (37) ablation. Another 33 patients were enrolled according to the protocol without concomitant ablation (NoMaze group). All patients had an echocardiogram before and one year after surgery. Results The 42 ablated patients who recovered sinus rhythm (SR) one year after surgery had comparable left and right systolic ventricular function and a mean reduction of LA size by 23% (LA volume index decreased from 66±24 to 54±13ml/m2, p=0.004). The 3DE extracted reservoir and booster function showed higher values after RF ablation compared to Cryo (37±10% vs 26±6%; p<0.001 resp. 18±9 vs 7±4%, p<0.001), while passive conduit function was comparable (24±11 vs 20±8%; p=0.17). Patients with restored SR without ablation had more preserved systolic atrial function but similar LA size reduction as those who underwent maze surgery. Conclusions SR restoration after MV surgery and maze results in LA size reduction irrespective of the energy source used. By 3DE we could show that, compared to RF maze, the extension of ablation area produced by Cryo implies a major LA structural remodeling affecting LA systolic function. The reverse size remodeling seems to be the least, the longer AF history. Funding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): ALF funding for clinical research-Sweden