Background: Not infrequently, chorda tendinae rupture (CTR), which was not recognized preoperatively by echocardiography, was found during mitral valve (MV) surgery in patients with MV prolapse. However, predictors of unrecognized CTR in MV prolapse remain to be determined. The purpose of this study was to evaluate predictors of echocardiographically unrecognized CTR in patients with severe mitral regurgitation (MR) due to MV prolapse. Methods: Between January 2005 and December 2009, we enrolled 124 consecutive patients who underwent MV replacement or repair for severe MR due to non-rheumatic MV prolapse. MR due to infective endocarditis, ischemic heart disease, or echocardiographically detected CTR was excluded. Transthoracic and transesophageal echocardiography were performed in all patients. The study population was divided into 2 groups; group 1, surgically proven CTR (n=51, age 54짹13 years, male 57%), and group 2, no CTR (age 52짹15 years, male 52%). Results: Echocardiographically unrecognized CTR was found in 51(41%) of 124 patients who were undergoing MV surgery due to MR. It was more common in patients with posterior or single leaflet prolapse. Although the severity of MR and left ventricular ejection fraction were greater in patients with unrecognized CTR, atrial fibrillation was less common (16% vs. 38%) and left atrial volume index (LAVI) was smaller compared to those without (51.5짹19.9 ml/m² vs. 69.0짹47.3 ml/m²). In multivariate analysis, involvement of posterior leaflet (OR 2.80, 95% CI 1.15 to 6.84), single leaflet (OR 3.18, 95% CI 1.07 to 9.45), severity of MR (OR 4.76, 95% CI 1.96 to 11.59), and LAVI (OR 0.98, 95% CI 0.96 to 0.99) were independently associated with the presence of CTR (p<0.05 for all). Conclusion: Echocardiographically unrecognized CTR is not uncommon in patients who were undergoing MV surgery due to MV prolapse. Posterior or single leaflet involvement, severity of MR, and smaller left atrial volume are independent predictors of this condition.