Background: We investigated how diabetes mellitus (DM) affects the outcome of acute ischemic stroke (AIS), comparing with the outcomes in those who had hypertension (HT) and atrial fibrillation (AF). Methods: This study was a subanalysis of PROTECT4.5, which was previously performed as a large-scale, prospective observational study of edaravone with approximately 10,000 patients with AIS in Japan. The study patients treated with edaravone alone or edaravone + alteplase (recombinant tissue plasminogen activator [tPA]) were analyzed for their outcomes and explored for the risk factors of poor outcome, after being divided into 8 groups according to their affected complications of DM, HT, or AF in the groups treated with edaravone alone or edaravone + tPA. Results: Among patients treated with edaravone alone and edaravone + tPA, the mean reduction in the National Institutes of Health Stroke Scale from baseline to 3 months after the onset was 2.0 and 4.4 in DM groups, respectively. The reduction was smaller in these groups compared with other groups (3.3-4.3 and 6.0-7.7, respectively). The logistic regression model revealed that DM was an independent risk factor for highly unfavorable outcome of modified Rankin Scale score 3-6 at 3 months after the onset, among both patients treated with edaravone alone and those treated with edaravone + tPA (odds ratio [OR]: 2.23, 95% confidential interval [CI]: 1.42-3.50 and OR: 2.05, 95% CI: 1.33-3.14, respectively). Conclusions: DM is suggested to adversely affect the outcome of AIS in Japanese patients. (c) 2018 The Authors. Published by Elsevier Inc. on behalf of National Stroke Association.