Backgrounds and Purposes : Lung gas exchange is a critical step in cardiopulmonary homeostasis. The efficiency of the gas exchange can be assessed by the carbon monoxide diffusing capacity (DLCO) test. Many studies showed that DLCO is decreased in both systolic and diastolic heart failure. However, correlation between atrial rhythm and DLCO was rarely reported. We hypothesized that there would be more reductions of DLCO in atrial fibrillation (AF) patients compared with sinus rhythm (SR) patients. We aimed to evaluate the importance of atrial rhythm on alveolar gas exchange capacity. Patients and Methods : Among 60 patients recruited, 31 patients were assigned to SR group and 29 patients were assigned to AF group. BNP and hs-CRP were measured and echocardiography and pulmonary function test (PFT) with DLCO test were performed. Patients with systolic heart failure (LV ejection fraction < 50% ), amiodarone treatment, idiopathic pulmonary fibrosis, obstructive airway disease (FEV1/FVC < 70%), tachycardia (heart rate ��� 100 /min) and acute infectious disease were excluded. Results : The mean age (SR 52.3 짹 10.7 vs AF 59.6 짹 10.6 years, p=0.011) were significantly older in AF group than SR group. Left atrial diameter was significantly larger at AF group (SR 3.6 짹 0.7 vs AF 4.2 짹 0.7 cm, p= 0.001) than SR group. The early diastolic transmitral velocity (E) was significantly higher in AF group (SR 68.1 짹 18.8 vs AF 81.8 짹 17.8 cm/sec, p= 0.006). BNP was significantly elevated at AF group (SR 50.37 짹 104.18 vs AF 133.6 짹 130.0 pg/ml, p=0.009). In diffusion test, DLCO percent predicted value was significantly decreased at AF group (SR 81.1 짹 18.9 vs AF 70.3 짹 14.6%, p=0.017). DLCO/VA (alveolar volume) percent predicted value was significant decreased at AF group (SR 102.7 짹 20.4 vs AF 92.4 짹 18.7 %, p=0.045). In all patients, DLCO percentage of predicted value and BNP were negatively correlated (r=-0.472, p=0.000). Conclusion : In this study, DLCO were decreased in AF group than SR group. This finding implicates that AF may worsen dyspnea in diastolic heart failure, therefore sinus conversion could improve dyspnea in diastolic heart failure with AF.