The development of mitral regurgitation (MR) after an acute myocardial infarction (AMI) is a recognized and frequent complication and its negative impact on survival has been observed. However, few data exist regarding MR after non-ST-segment elevation acute coronary syndrome (NSTSEACS). Our aim was to investigate the incidence, clinical predictors, and prognostic implications of MR in the setting of NSTSEACS. We studied 300 consecutive patients (71.7% men, mean age 66.9+/-13 years) admitted to our coronary care unit for an NSTSEACS. Every patient underwent an echocardiographic study during the first week after the index NSTSEACS and was clinically followed up. MR was detected in 42% (126 patients; 88 men, mean age 71.3+/-11 years). Mean follow-up was 425.6+/-194.8 days. Only age and left ventricular (LV) ejection fraction (EF) were found as independent markers of the development of MR; no variable was found as an independent predictor of in-hospital mortality and only MR was found as an independent predictor of long-term outcome. MR is frequent after an NSTSEACS. Age and a low LV EF are factors associated to its development. The presence and degree of MR confer a worse long-term prognosis to patients after a first NSTSEACS. Thus, the presence of MR should be specifically assessed in every patient after an NSTSEACS.