The objective of this study was to clarify the left atrial (LA) reservoir and booster pump function in patients with left ventricular (LV) diastolic dysfunction. To determine LA reservoir and booster pump function, a new algorithm to determine LA functional volume change curve (FVC) was developed from Doppler flow spectra of pulmonary venous flow and LV inflow by transthoracic echocardiography in 110 patients. Patients were classified into normal (N), and abnormal (AB) and pseudonormal (PN) groups on the basis of their Doppler flow patterns. From the indices of FVC, atrial reservoir volume (ARV), passive emptying volume (PEV) and active emptying volume (AEV) were obtained. ARV/stroke volume (SV) was increased in the AB group, but decreased in the PN group compared with N (N, 0.61 ±0.09; AB, 0.73±0.10; PN, 0.52 ±0.13, p<0.05). PEV/SV was significantly decreased in AB, but increased in PN compared with N (N, 0.27±0.07; AB, 0.19±0.07; PN, 0.31±0.18, p<0.05). AEV/SV was significantly increased in AB, but decreased in PN compared with N (N, 0.41±0.08; AB, 0.56±0.10; PN, 0.26±0.19, p<0.05). Thus, in patients with an abnormal relaxation pattern, the LA reservoir and booster pump function are augmented, but in patients with a pseudonormal pattern, both LA reservoir and booster pump function are deteriorated, suggesting a vulnerability to pulmonary congestion. (Jpn Circ J 2001; 65: 953 - 957)