Abstract： Objective To investigate the correlation between electroencephalography (EEG) approximate entropy(ApEn)of the lesion and National Institutes of Health Stroke Scale(NIHSS)score in patients with ischemic stroke within 24 h from onset. Methods From December 2014 to March 2016,37 patients with ischemic stroke admitted to our hospital were enrolled in the study. For these patients,NIHSS scores were recorded within 24 h from onset,and EEG performed to calculate ApEn of different leads based on international 10-20 system for scalp electrode placement. The patients were grouped according to the scalp electrodes which correspond for the sites of lesion as indicated by magnetic resonance imaging(MRI). The correlation between ApEn and NIHSS score was analyzed in each group. Results A total of 37 patients were divided into groups G1 through G6, according to electrodes corresponding to MRI-indicated lesions which accounted for the left anterior frontal region , left middle frontal region,left middle temporal region,left parietal region,right parietal region,and left lateral occipital region. Spearman correlation analysis showed that ApEn was negatively correlated with NIHSS score for lesions supplied by the anterior circulation(groups G1 through G5)(rs=-0.252,-0.038,-0.270,-0.387,-0.300, all P<0.05),but not for the lesions supplied by the posterior circulation(group G6)(P>0.05). Conclusion ApEn can be used as an indicator of disease severity in ischemic stroke of regions supplied by anterior circulation.