Objective To investigate the changes and significance of serum cyclic RNA TTC3(circTTC3)and microRNA-138-5p(miR-138-5p)levels in elderly patients with acute cerebral infarction(ACI).Methods From September 2021 to September 2023,the Geriatric Medicine Center/Cadre Medical Department of West China Hospital of Sichuan Univer-sity treated 128 elderly patients with ACI(ACI group),according to the National Institute of Health Stroke Scale(NIHSS),elderly ACI patients were separated into a mild group of 42 cases,a moderate group of 49 cases,and a severe group of 37 cases,meantime,120 healthy individuals who came to the outpatient clinic for physical examination were regarded as a control group,real-time fluorescence quantitative PCR method was applied to determine the expression levels of circTTC3 and miR-138-5p in serum,Pearson method was applied to analyze the correlation between serum circTTC3 and miR-138-5p in elderly ACI patients,Spearman method was applied to analyze the correlation between serum circTTC3 and miR-138-5p levels and NIHSS scores in elderly ACI patients,ROC curve was applied to analyze the diagnostic value of serum circTTC3 and miR-138-5p expression levels in elderly ACI patients.Results Compared with the control group,the expression level of circTTC3 in serum of ACI group was increased,and miR-138-5p was decreased(t/P=17.207/<0.001,16.239/<0.001).The serum circTTC3 level in mild subgroup was lower than that in moderate subgroup than that in severe subgroup,and the ex-pression level of miR-138-5p in mild subgroup was higher than that in moderate subgroup than that in severe subgroup(F/P=26.579/<0.001,105.935/<0.001).Serum circTTC3 expression level in elderly ACI patients was positively correlated with NIHSS score(r=0.521,P<0.001),and negatively correlated with MCA and ACA(r=-0.425,-0.392,all P<0.001).The expression level of miR-138-5p was negatively correlated with NIHSS score(r=-0.785,P<0.001),and positively correlated with MCA and ACA(r=0.571,0.635,all P<0.001).The AUCof circTTC3,miR-138-5p and combined Mir-138-5P in the diag-nosis of ACI nerve defect was 0.817,0.810 and 0.878,respectively,and the combined diagnosis value was better than that of single diagnosis(Z=2.106,2.406,P=0.035,0.016).Conclusion The expression levels of circTTC3 and miR-138-5p in the se-rum of elderly ACI patients increase,and the expression levels of miR-138-5p decrease.The two are closely related to the severity of condition and may serve as targets for the diagnosis and treatment of ACI.