Abstract: Objective  To investigate the effect of systemic immune-inflammation index(SII) based on neutrophil, platelet and lymphocyte on the clinical prognosis of patients with small cell lung cancer(SCLC). 
Methods  The case data of 98 patients with SCLC were analyzed retrospectively. According to the results of blood routine before treatment, the SII value was calculated, and the optimal cut-off value of SII was obtained by ROC curve. The effects of SII level and clinicopathological characteristics on progression-free survival(PFS) and the overall survival(OS) were estimated using the Kaplan-Meier curves and Cox proportional hazards regression models. 
Results  The optimal cut-off value of SII was 571.5. The proportion of the low SII group in the limited stage was higher than that of the high SII group(P＜0.05). The median PFS and OS of the low SII group were longer than that of the high SII group(P＜0.001). Multivariate analyses further revealed SII level as an independent prognostic factor for PFS and OS(P＜0.05). 
Conclusion  High SII often indicates poor prognosis and plays an important role in prognostic evaluation of SCLC patients. 

Key words: small cell lung carcinoma, systemic immune-inflammation index, prognosis