To assess whether the presence of mitral regurgitation has a protective effect on left ventricular thrombus formation in a heterogeneous group of patients with dilated cardiomyopathy, a group of 103 patients with dilated cardiomyopathy identified by means of echocardiographic criteria was assembled over 1 year. The purpose of the study was to define a subgroup of patients with dilated cardiomyopathy from whom long-term anticoagulation might be withheld. Each echocardiogram was evaluated for the presence of left ventricular thrombus, presence and severity of mitral regurgitation, and ejection fraction. The role of clinical factors and clotting factors in left ventricular thrombus formation was assessed. Left ventricular thrombus was not present in 91 patients (group A) and was present in 12 patients (group B). Group B had larger left atrial and left ventricular systolic dimensions and decreased left ventricular systolic function. Mitral regurgitation jet area and ratio between mitral regurgitation jet area and left atrial area were lower (signifying less severe mitral regurgitation) among patients in group B. Although mitral regurgitation was equally present in group A and group B, severe mitral regurgitation was found only in group A patients ( 11 of 91 patients). Among patients with dilated cardiomyopathy, left ventricular ejection fraction is the factor most associated with left ventricular thrombus formation. The presence of severe mitral regurgitation may have a protective role in left ventricular thrombus formation.