Background: It has been known that restoring sinus rhythm in AF patients is very important factor to improve left ventricular (LV) and left atrial (LA) function. We hypothesized that the LV dysfunction patients who can restore sinus rhythm after AF ablation will show better LV and LA function. Methods: From 2001 to 2007, 158 patients (M:F=99:59, mean age 57짹10yrs, paroxysmal AF: chronic persistent AF=103:55) were enrolled. Among them 22 patients showed decreased LV systolic function (EF<45%) and had drug refractory AF. We divided the patients into 4 groups; paroxysmal and persistent AF with recurrence and non-recurrence groups and compared the cardiac function by transthoracic echocardiography (TTE). For the patients with decreased LV systolic function, we compared cardiac function before and after restoration of sinus rhythm. All TTE were followed up in more than two months after AF ablation. Results: 1.18 of 22 patients with decreased LV systolic function returning to sinus rhythm after AF ablation showed significant increase of ejection fraction (41.7짹3.1 vs 61.3짹3.1 %, p<.001) , compared to before ablation. LA size did not show significant difference (p>.05). 2.In paroxysmal AF group, 24(23.3%) patients showed recurrence. The changes of LV ejection fraction (5.7짹5.4 vs 6.0짹6.0 %) and fractional shortening (6.5짹7.5 vs 7.1짹5.5 %) showed only trend of increase in non-recurrence than recurrence group, but it was statistically insignificant (p=.888, p=.785). The changes of LA volume (-0.1짹17.6 vs -0.3짹15.2 ml) also trended to decrease in in non-recurrence than recurrence group, but insignificant (p=.828). 3.In persistent AF group, 15(28.8%) patients showed recurrence. The changes of LV ejection fraction (7.4짹6.1 vs 8.6짹7.3 %) and fractional shortening (6.9짹8.9 vs 8.2짹7.1 %) trened to increase in non-recurrence than recurrence group, but insignificant, respectively (p=.581, p=.646). The changes of LA volume (-0.5짹1.2 vs -1.6짹7.6 ml) trended to decrease in in non-recurrence than recurrence group, but insignificant (p=.813). Conclusions: Catheter Ablation of AF can be very effective treatment in drug refractory AF patients who have serious LV dysfunction.