Aims: Functional mitral regurgitation(MR) and myocardial asynchronism occur commonly in patients with dilated cardiomyopathy and affect adversely their prognosis and symptoms. The aim of this study was to evaluate the mechanisms of changes in MR severity during dynamic exercise in patients with chronic heart failure(CHF). Methods and results: Seventy patients with CHF due to left ventricular(LV) systolic dysfunction [LV ejection fraction(EF)< 40% ] and functional MR were studied. All were in sinus rhythm. Medications were left unchanged for the study. Each patient performed a maximal symptom-limited exercise test with continuous 2D-Doppler echocardiography. Mitral regurgitant volume(RV) and effective regurgitant orifice(ERO) were determined at rest and during exercise. LV asynchrony using Doppler tissue imaging and interventricular asynchrony using conventional pulsed-Doppler were evaluated at rest. Resting LV EF averaged 25± 8% . Mean resting LV and interventricutar mechanical delays were 56± 50 and 43± 37 ms, respectively. The overall median values for mitral ERO and RV did not significantly change during dynamic exercise(11 [7-16] vs. 11 [6-21] mm2 and 14 [10- 22] vs. 12 [9-23] mL, respectively). However, changes in mitral ERO and RV were individually variable and significantly correlated with the degree of LV asynchronism(r=0.66, P< 0.0001 and r=0.66, P< 0.0001, respectively). Conclusion: Changes in MR are variable during dynamic exercise. LV asynchronism at rest substantially contributes to worsening of functional MR during dynamic exercise in patients with CHF due to LV systolic dysfunction. Aims: Functional mitral regurgitation (MR) and myocardial asynchronism occur commonly in patients with dilated cardiomyopathy and affect adversely their prognosis and symptoms. The aim of this study was to evaluate the mechanisms of changes in MR severity during dynamic exercise in patients with chronic heart failure (CHF). Methods and results: Seventy patients with CHF due to left ventricular (LV) systolic dysfunction [LV ejection fraction (EF) <40%] and functional MR were studied. All were in sinus rhythm. Medications were left unchanged for the study. Each patient performed a maximal symptom-limited exercise test with continuous 2D-Doppler echocardiography. Mitral regurgitant volume (RV) and effective regurgitant orifice (ERO) were determined at rest and during exercise. LV asynchrony using Doppler tissue imaging and interventricular asynchrony using Resting LV EF averaged 25 ± 8%. Mean resting LV and interventricutar mechanical delays were 56 ± 50 and 43 ± 37 ms, respectively. The overall median values ​​for mitral ERO and RV did not change during dynamic exercise (11 [7-16] vs. 11 [6-21] mm2 and 14 [10-22 ] vs. 12 [9-23] mL, respectively). However, changes in mitral ERO and RV were individually variable and significantly correlated with the degree of LV asynchronism (r = 0.66, P <0.0001 and r = , respectively). Conclusion: Changes in MR during variable during dynamic exercise. LV asynchronism at rest substantially contributes to worsening of functional MR during dynamic exercise in patients with CHF due to LV systolic dysfunction.