Diastolic mitral regurgitation is a type of functional mitral regurgitation that develops via a reversal of the left atrioventricular pressure gradient during diastole. This study aimed to explore the mechanism underlying early diastolic mitral regurgitation (EDMR) in patients with left ventricular (LV) aneurysms after anterior myocardial infarction (AMI) by assessing the intraventricular pressure difference using vector flow mapping. We enrolled 23 consecutive patients with LV aneurysms (with and without EDMR) and 15 healthy men as controls. In the control group, LV suction began from the apex during early diastole. In contrast, the blood that pooled in the apical aneurysm during systole generated a relatively higher pressure at the apex than at the basal LV during early diastole; consequently, the pressure reversal phenomenon occurred in the LV. Compared to the EDMR– group, the EDMR + group (n = 7) exhibited a significantly higher diastolic time ratio ([time from the second heart sound to the pressure inversion point]/[total diastolic time]) (P < 0.001). The diastolic time ratio was significantly correlated with log BNP, but not with E/A, E/E′, or the left atrial expansion index. In conclusion, EDMR in LV aneurysm may be due to a prolonged diastolic time ratio leading to prolonged pressure inversion in the LV during early diastole.