BACKGROUND The aims of this study were to elucidate the incidence of mitral regurgitation during off-pump coronary artery bypass (OPCAB) surgery to evaluate the relationship of the changes with ventricular function and possible mitral valve regurgitation caused by positioning. METHODS Included in the study were 60 consecutive patients who underwent CAB grafting on the beating heart. We monitored several hemodynamic variables (systolic arterial pressure, mean arterial pressure, right atrial pressure, pulmonary capillary wedge pressure, and heart rate) at baseline and after each anastomosis and used transesophageal echocardiography (TEE) routinely after sternotomy, during each anastomosis, and after completion of the operation. Valvular functions, ejection fraction, and wall motion systolic index were recorded during each TEE evaluation. RESULTS All of the patients underwent complete revascularization. We performed 132 consecutive OPCAB anastomoses in 60 patients (60 left anterior descending artery [LAD], 20 right coronary artery [RCA], 45 left circumflex coronary artery [LCX], and 7 diagonal artery grafts). During LCX anastomosis, 38 (84.4%) of 45 patients developed moderate mitral regurgitation. The wall motion score index (WMSI) significantly increased during CX grafting, as was demonstrated by higher WMSI values than for the RCA, diagonal, and LAD grafts. The ejection fraction was decreased significantly during CX and RCA anastomoses compared with baseline levels. The hemodynamic changes were in accord with these findings. The greatest hemodynamic compromise was seen during CX anastomosis. CONCLUSION Positional mitral regurgitation occurs frequently and is a major contributor to hemodynamic instability during posterior- and lateral-wall revascularization during the OPCAB procedure.