Objective:To investigate the predictive value of systemic inflammatory response index (SIRI) for early neurological deterioration (END) and clinical outcome in patients with branch atherosclerotic disease (BAD).Methods:Consecutive patients with BAD admitted to the Department of Neurology, Jiangyin People’s Hospital Affiliated to Southeast University from September 2021 to September 2022 were retrospectively included. The clinical data were collected and SIRI was calculated. The calculation method of SIRI was neutrophil count × monocyte count/lymphocyte count. END was defined as an increase of ≥2 in the total score of the National Institutes of Health Stroke Scale (NIHSS) or an increase of ≥1 in the motor function score within 1 week of onset. The modified Rankin Scale was used for outcome evaluation at 3 months after onset, with a score >2 were defined as poor outcome. Multivariate logistic regression analysis was used to determine the independent correlation between SIRI and END, as well as poor outcome. Receiver operating characteristic (ROC) curve was used to evaluate the predictive value of SIRI for poor outcome. Results:A total of 125 patients with BAD were included, of which 62 (49.6%) had END and 32 (25.6%) had poor outcome. The multivariate logistic regression analysis showed that intravenous thrombolysis (odds ratio [ OR] 1.083, 95% confidence interval [ CI] 1.082-1.240; P=0.043) and high SIRI ( OR 1.465, 95% CI 1.150-3.676; P=0.028) were independent risk factors for END in patients with BAD; END ( OR 1.130, 95% CI 1.032-1.384; P=0.006), high baseline NIHSS score ( OR 1.571, 95% CI 1.184-2.101; P=0.003) and high SIRI ( OR 2.062, 95% CI 1.152-3.672; P=0.01) were independent risk factors for poor outcome in patients with BAD. ROC curve analysis showed that the area under the curves for SIRI, baseline NIHSS score, and the both combined prediction of poor outcome were 0.66 (95% CI 0.54-0.78), 0.70 (95% CI 0.58-0.81), and 0.83 (95% CI 0.74-0.93), respectively. Conclusions:High SIRI is an independent risk factor for END and poor outcome in patients with BAD. The SIRI and baseline NIHSS scores have certain predictive value for poor outcome in patients with BAD, and their combined diagnostic value is higher.