Background: Left atrial (LA) volume is a prognostic index in chronic mitral regurgitation (MR). However, little is known about LA function in this setting. We hypothesized that LA dysfunction is related to pulmonary hypertension in chronic MR. Methods: Seventy-one patients with organic chronic MR who underwent real-time 3-dimensional transthoracic echocardiography (RT3DE) were studied. Left atrial volumes and peak passive and active LA emptying rates were obtained.Total LA emptying fraction was calculated as follows: [(maximum−minimum LA volume)/maximum LA volume] × 100. Similarly,active and passive LA emptying fractions were calculated. From transmitral flow, the peak early (E) and late (A) diastolic filling velocities and E/A ratio were obtained. The early (E′) and late (A′) diastolic myocardial velocities were obtained by tissue Doppler interrogation of mitral annulus.ResultsEffective regurgitant orifice area (EROA) was 0.57 ± 0.29 cm2. Right ventricular systolic pressure (RVSP) wasmeasured in 57 patients and averaged 37 ± 13 mm Hg. Patients with MR and high RVSP displayed higher minimum LA volume,E/A ratio, E/E′ratio, EROA, and MR volume, and lower A′velocity, peak active LA emptying rate, active LA emptying fraction,and total LA emptying fraction than patients with MR and normal RVSP. Multiple regression analysis revealed that EROA(r= 0.51,P= .01) active LA emptying fraction (r=−0.53,P= .02), E/E′ ratio (r= 0.50;P= .04), and the lateral A′ velocity(r=−0.46; P= .003) were independently correlated with RVSP. Conclusions: Left atrial function determined by RT3DE had significant correlation with RVSP in chronic MR, irrespective of MR severity. Thus, pulmonary hypertension in chronic MR may depend not only on MR severity but also on LA function.