Backgrounds. Alveolar gas diffusion is impaired in systolic heart failure (HF). Lung diffusion for carbon monoxides (DLCO) is decreased in systolic HF. In diastolic HF, left ventricular end diastolic pressure and left atrial pressure are increased and it can decrease lung diffusion capacity also. Diastolic HF is an independent predictor of atrial fibrillation (AF), and after development of AF patient���s condition are usually more deteriorated. In this study, we test the hypothesis that there would be more reductions of DLCO in AF patients compared with sinus rhythm patients with diastolic HF Methods and results. We performed echocardiography and pulmonary function test with DLCO measured with the single breath constant expiratory technique in 35 patients,(17 controls 59.5짹9.9 yrs, 18 AF patients 57.6짹12.2 yrs). DLCO data are reported as percentage of predicted. Patients with use of amidarone were excluded. Results. Ejection fraction (EF) (sinus 67.5짹10.7, AF 61.0짹14.5%), E/E��� (sinus 8.2짹2.7 AF 8.9짹2.4) were not different between 2 groups. In DLCO, there is no significant difference between sinus and AF patients (sinus 89.3짹26.9, AF 86.5짹18.8 ,p=0.73). LA diameter was significantly different between AF and controls (control 3.95짹0.7, AF 4.5짹0.6, p=0.023) Based on simple linear regression analysis, DLCO and BNP were negatively correlated. (r= -0.548, p=0.001). DLCO and EF were positively correlated.(r=0.407, P=0.017) Conclusions. In this study, we have observed gas diffusion is negatively correlated with BNP and positively correlated with EF. But there is no difference of DLCO between AF and sinus rhythm, which suggests contribution of atrial function to alveolar gas exchange may be not very significant in diastolic HF.