Residual mitral regurgitation (MR) after repair is a risk factor for late reoperation. The use of intraoperative transesophageal echocardiography (IOTEE) decreases the incidence of immediate repair failure. This study identifies the mechanisms of immediate failure by IOTEE in the quadrangular resection technique, a well-standardized mitral valve repair procedure to guide further repair procedures. Two hundred five consecutive patients underwent quadrangular resection due to prolapse or flail posterior leaflet. Twenty-four patients (11%) had immediate failure. Immediate reinstitution of cardiopulmonary bypass ("second pump run") was needed in 21 patients (10%) for further repair. The identified mechanisms of failure were residual cleft provoking interscallop malcoaptation into the posterior leaflet in 8 patients, residual prolapse of the anterior or posterior leaflets in 1 and 4 patients, respectively, residual annular dilation in 3, left ventricular outflow obstruction in 2, suture dehiscence in 2, and other mechanisms in another 2 patients. In 20 patients (95%), IOTEE guided further repair with resolution of the residual MR, whereas 1 patient underwent valve replacement due to pharmacologically untreatable left ventricular outflow obstruction. In conclusion, even if this type of valve repair technique is well standardized, the incidence of immediate failure is not negligible. IOTEE identified the mechanisms of the immediate failure and guided further repair procedures, thus reducing the incidence of valve replacement (0.5%) without increasing perioperative mortality and morbility.