Objective: To find the relationship between NIHSS score and ischemic lesions on magnetic resonance imaging in acute ischemic stroke due to middle cerebral artery occlusion within 24 hours of symptom onset. Methods: a prospective, observative research on 48 patients with MCA occlusion in Emergency Department, Bach Mai Hospital from January to August, 2011. Results: Patients with M1 and M2 occlusions had NIHSS score average of 23,2 + or - 7,29 and 11,4 + or - 3,69, respectively, ischemic lesions on MRI were 105,48 + or - 37,217 cm3 and 48,76 + or - 30,730 cm3, respectively. Conclusions: NIHSS score has a closely linear relationship with ischemic lesions on MRI.