Surgical valve repair for mitral regurgitation has significant advantages over valve replacement, but little is known about the mechanisms of its failure. This echocardiographic study examined abnormalities leading to failed mitral valve repair in two populations: "immediate failure" of the valve repair in the operating room requiring a second run of cardiopulmonary bypass and "late failure" of valve repair necessitating reoperation on another occasion. Intraoperative echocardiography (IOE) after cardiopulmonary bypass was performed in 309 patients undergoing valve repair for mitral regurgitation over a 3-year period. Twenty-six (8%) of these patients had immediate failure of the repair demonstrated by IOE, requiring further repair or replacement during the same thoracotomy. The causes of immediate failure were left ventricular outflow tract obstruction (10 patients), incomplete correction (10 patients), and suture dehiscence (six patients). Echocardiography was performed on 17 patients requiring reoperation for recurrent mitral regurgitation who had undergone previous primary valve repair. These late failures resulted from progressive degenerative leaflet or chordal disease (n = 9) or suture dehiscence of the annular ring or the leaflet resection site (n = 6). In only two patients early in the series did the problem originate from inadequate initial surgery. IOE is an effective marker for unsuccessful mitral valve repair, and affords an understanding of the mechanism of the persistent dysfunction. Immediate failure of mitral repair may be reduced by greater attention to the mechanism of valve dysfunction and by changes in valvuloplasty technique to avoid outflow tract obstruction. Late failure after mitral repair occurs predominantly due to progression of disease, particularly in patients with severe myxomatous or annular abnormalities that are prone to progress.