Background and objectives: Atrial dilatation contributes to the inducibility of atrial fibrillation(AF) by changing atrial electrophysiological properties. None of the data was collected from the left atrium(LA), where the most significant electrophysiological impact of mitral stenosis might be expected to occur. We investigated electrophysiological changes after a reduction of LA pressure in patients undergoing percutaneous balloon mitral commissurotomy(PBMC). Methods: Twenty-six patients(21 wome, 50짹12 yo) of mitral stenosis including 6 with normal sinus rhythm were studied. Effective refractory period(ERP), monophasic action potential(MAPD90) and conduction time(CT) were measured simultaneously in both right atrium(RA) and LA at 600, 500, 400 and 300ms of drive cycle length(DCL). Restitution slope and AF cycle length(AFCL) in case of AF were also calculated. All measurements were repeated after PBMC. Results: Mean LA pressure was significantly reduced after PBMC (21.5짹6.9 mmHg versus 11.0짹1.8 mm Hg; P<0.05). There were homogenous, although not significant, increases in regional ERP after PBMC, which was more prominent within the LA.(212짹32 ms versus 241짹41ms;P=0.1) The increase of MAPD90 after PBMC was significant in the LA at £ 500ms DCL, but not in the RA. The CT also decreased significantly at all the tested DCLs��� after PBMC. Changes in LA pressure were significantly correlated with the change of LA MAPD90. AF induced in 3 of 6 sinus rhythm patients during single atrial stimulation, which was abolished after PBMC. However, no significant change of AFCL or slope after PBMC were observed in AF patients. Conclusion: Chronic atrial stretch altered atrial electrophysiological milieu, especially of the LA, which was partially reversible. This result support the theoretical basis for early intervention to reduce atrial pressure overload.