Background Obstructive sleep apnea (OSA) is closely associated with atrial fibrillation, which is provoked by electrical and structural remodeling. However, the association between OSA and atrial remodeling has not been fully elucidated. Methods and Results Atrial electromechanical activation time (EMAT) was investigated using tissue Doppler imaging (TDI) in men with severe OSA (n=24) and control subjects (n=24). The EMAT was determined as the time interval from the initiation of P-wave deflection until the peak of local lateral left atrial (LA) TDI signal. The early diastolic velocity of the mitral annulus (E_a) and the EMAT were significantly lower and longer in OSA cases than in controls (E_a: 6.1±0.9 cm/s vs 7.3±1.5 cm/s, P=0.001; EMAT: 129.7±11.5 ms vs 118.5±12.3 ms, P=0.002). Among OSA cases, the apnea - hypopnea index (AHI) was significantly correlated with EMAT (r=0.660, P<0.001), E_a (r=-0.609, P=0.002), LA dimension (r=0.486, P=0.016), and early diastolic velocity of mitral flow (E)/E_a ratio (r=0.418, P=0.042). In multivariate stepwise linear regression analysis, EMAT was independently associated with AHI (P=0.025) and E_a (P=0.028) in OSA cases. Conclusion EMAT measured by TDI could be a useful parameter for identifying atrial remodeling in patients with severe OSA. (Circ J 2009; 73: 249 - 255)