Background Functional mitral regurgitation (MR) is common in patients with chronic heart failure (CHF) and left ventricular dysfunction. The severity of CHF is expressed in terms of exercise tolerance, so MR during exercise would affect the severity of heart failure. However, it is not well known how much MR increases during exercise, or if it is related with severity of heart failure. Methods and Results Seventeen subjects underwent dynamic cycle exercise at steady-state levels of 80% and 150% of anaerobic threshold (AT). During each exercise level, the MR jet and left atrial (LA) area were measured, and the degree of MR was expressed as the ratio (MR/LA). The MR/LA increased slightly at 80% AT (rest: 15.5±7.8%, 80% AT: 21.7±9.3%, p<0.05). It increased more at 150% AT (29.2±11.6%, p<0.01). The MR/LA at both 80% and 150% AT weakly correlated with peak oxygen consumption/heart rate (r=-0.509). They showed a weakly positive correlation with the ventilatoly equivalent/CO₂ production slope (r=0.340). Conclusions MR during exercise increases as stroke volume worsens during exercise, so evaluation of MR during exercise is important in the management of patients with CHF. (Circ J 2006; 70: 1563 - 1567)