Abstract Background Atrial fibrillation (AF) is associated with impaired mechanical function of the atria. The restoration of sinus rhythm (SR) leads to improvement of mechanics, however the onset of those changes is not established yet. Purpose To analyse changes in left atrial (LA) mechanical function after conversion to SR. Methods Forty patients (mean age 65 ± 12 years, 63% male) with nonvalvular AF underwent successful electrical cardioversion. Transthoracic echocardiography with speckle tracking analysis (STE) of LA was performed in AF and within 24 hours after cardioversion, in SR. We analysed standard parameters of left ventricle (LV) and LA. By the use of STE we assessed peak atrial longitudinal strain (PALS), time to PALS indexed to RR interval (TT-PALS/RR) and intra-atrial asynchrony. Results Heart rate during echocardiography was higher before cardioversion - median (IQR) 90 beats per minute (80-110) vs 61 (56-72); p < 0.0001. The mean LV ejection fraction in our study group was 48 ± 11%. We noticed significant improvement in LA STE measurements and in early diastolic (E’) mitral annular velocity. There were no significant changes in systolic (S’) mitral annular velocity and in the ratio of transmitral Doppler early filling velocity to E’ (E/E’). Detailed results are present in the table. Conclusions Speckle tracking measurements are able to detect early mechanical changes in LA even within 24 hours of SR. Atrial fibrillation Sinus rhythm p S" (cm/s) 6 (5-6.5) 6 (5-6.6) NS E" (cm/s) 10 (7.5-13) 9.5 (6.8-10.5) 0.0008 E/E" 8.2 (6.3-12.3) 9.3 (7.5-13) NS PALS (%) 9.9 ± 4.6 14.5 ± 6 <0.0001 TTP-PALS/RR (%) 51 (44-58) 42 (38-45) 0.0001 LA asynchrony 15 (11-19) 9 (8-14) 0.0007