The aim of the present study was to devise a simple grading scale for assessing the risk of development of malignant MCA infarction (MMI).Using MRI, patients with MCA infarction and proximal vessel occlusion (ICA or M1) within 24h of onset were retrospectively studied. MMI was defined as clinical deterioration, midline shift ≥5mm, or brain herniation within 48h of admission. We evaluated clinical factors independently associated with MMI and created a simple score according to the multivariate logistic regression analysis.Subjects comprised 119 patients, 57 of which (47.9%) developed MMI. Multivariate logistic regression analysis revealed the following independent factors associated with MMI: DWI ASPECTS ≤3 [odds ratio (OR), 4.16; 95% CI, 1.36–12.66, P=0.012], ACA territory involvement [OR, 6.90; 95% confidence interval [CI], 2.06–23.10, P=0.002], M1 susceptibility vessel sign (SVS) on T2*-gradient echo [OR, 4.55; 95% CI, 1.38–14.98, P=0.013], and hyperglycemia (glucose value≥145mg/dl) [OR, 5.31; 95% CI, 1.80–15.68, P=0.002]. These four variables were selected for use in the DASH score, with DWI ASPECTS ≤3 as 1 point, ACA territory involvement as 1 point, M1 SVS as 1 point, and hyperglycemia as 1 point. The likelihood of developing MMI for each score was as follows: score 0, 9.1%; score 1, 20.5%; score 2, 63.0%; score 3–4, 96.8%. The C statistic for the score was 0.88 (95% CI, 0.82–0.94, P<0.001).Our DASH score reliably assessed a risk for development of MMI in large MCA infarctions.