Abstract: AIM:To explore the effect of severe mitral regurgitation (MR) on left ventricular mural thrombus formation in patients with aneurysm after myocardial infarction (MI). METHODS: Clinical data of 340 consecutive patients with left ventricular aneurysm (LVA) after MI were retrospectively analyzed. Patients were divided into two groups according to the degree of mitral regurgitation diagnosed by echocardiography: 84 cases in severe MR group and 256 cases in nonsevere MR group. Mean left atrial (LA), LV end-diastolic and end-systolic diameter, thickness of the interventricular septum (IVS) and LV posterior wall (PW), LV ejection fraction (LVEF) and grade of the MR and left ventricular thrombus were compared between groups. RESULTS: The overall incidence of LV thrombus was lower in patients with severe MR than in patients without severe MR (11 vs. 22%, P<0.05). The mean LA and LV end-diastolic and end-systolic diameter were greater in patients with severe MR than in those without severe MR (P<0.01). The LVEF was lower in patients with severe MR than in patients without severe MR (P<0.01). No significant difference was found in the thickness of the IVS and LVPW between groups. CONCLUSION: Severe MR in patients with LVA after MI has some degree of preventive effect on LV mural thrombus formation. This relative risk reduction may be associated with an increased volumetric rate and diastolic volume overloading due to severe MR, which may overcome stagnation and a procoagulant state within the aneurysm.