Objective To study the expression of serum interleukin(IL)-17A,calcium binding protein S100A8 and S100A9 in children with severe Mycoplasma pneumoniae pneumonia(SMPP)and their prognostic significance.Methods A total of 116 children with SMPP who were diagnosed and treated in this hospital from March 2019 to March 2021 were enrolled as the SMPP group.According to the pediatric critical cases score,the SMPP children divided into non-critical group(43 cases),critial group(40 cases),extremely critical group(33 cases).According to the prognosis of 28 d after admission,the SMPP children were divided into a good prognosis group with 82 children and a poor prognosis group with 34 children.A total of 60 physical ex-amination of healthy children in the same hospital during the same period were enrolled as the control group.The levels of serum IL-17A,S100A8,S100A9,procalcitonin(PCT),C-reactive protein(CRP),IL-6 and tumor necrosis factor(TNF)-α were detected in each group.Pearson correlation analysis was used to analyze the cor-relation between serum levels of IL-17A,S100A8,S100A9 and PCT,CRP,IL-6,and TNF-α.Multivariate Lo-gistic regression analysis was used to analyze the factors affecting the poor prognosis of children with SMPP.The receiver operating characteristic(ROC)curve was used to analyze the value of each index in predicting the poor prognosis of children with SMPP.Results The SMPP group had significantly higher serum levels of IL-17A,S100A8,S100A9,PCT,CRP,IL-6,and TNF-α than the control group(P<0.05).In children with SMPP,the serum levels of IL-17A,S100A8,and S100A9 were positively correlated with PCT,CRP,IL-6,and TNF-α(P<0.05).The serum levels of IL-17A,S100A8 and S100A9 in extremely critical group were signifi-cantly higher than those in critical group and non-critical group(P<0.05).Elevated serum levels of IL-17A,S100A8 and S100A9 were independent risk factors for poor prognosis in children with SMPP.The area under the curve(AUC)of combined detection of serum IL-17A,S100A8 and S100A9 for predicting poor prognosis in children with SMPP was 0.895,which was higher than that of single detection of serum IL-17A,S100A8 and S100A9(0.833,0.764,0.810),the differences were all statistically significant(Z=3.780,6.723,5.012,P<0.059).The sensitivity and specificity of combined detection were 0.891 and 0.755,respectively.Conclu-sion The serum levels of IL-17A,S100A8 and S100A9 are increased in children with SMPP,which are related to the severity of SMPP.The combined detection of the three indicators has a high predictive value for the poor prognosis of SMPP.