To define the incidence of mitral regurgitation (MR) and elucidate its potential contribution to the development of severe congestive heart failure after acute mypcardlal infarction (AMI), Doppler echocardiograms were obtained within 48 hours of onset of AMI in 59 patients. The presence of MR was determined from the apical 4-chamber and parasternal long-axis views with pulsed Doppler. MR was detected in 23 of the 59 patients (39%) and was similarly frequent in patients with anterior (11 of 24 or 46%) and inferior AMI (12 of 34 or 35%). Patients with MR were older (71 ± 3 vs 62 ± 2 years, p < 0.005), had a higher incidence of prior AMI (8 of 23 vs 4 of 36, p < 0.05) and larger end-diastolic volume indexes by radionuclide ventriculography (112 ± 9 vs 72 ± 4, p < 0.005). A systolic murmur was heard in only 10 of 23 patients with MR detected by Doppler. Mortality determined 8 to 14 months after the index AMI was 48% (11 of 23) in patients with MR but only 11% (4 of 30) in those without it (p < 0.01). Thus, this study determined that clinically silent MR frequently complicates AMI and its presence is associated with and is a potential determinant of severe congestive heart failure and mortality.