Middle cerebral artery stenosis induces flow lateralization. However, the impact of stenosis degree on flow lateralization is not fully understood. This study investigated the impact of MCA stenosis severity on MCA blood flow lateralization using 4D flow MRI. Twenty-seven patients with symptomatic MCA stenosis were included. Absolute flow rates of MCA on stenosis and contralateral sides were obtained. Paired samples t-test was used to compare the flows on both sides. We found a significant difference between the flows in the severe stenosis group, while there was no significant difference in non-severe group.