Objective:To study lncRNA MEG3 expression and its relationship with Th17/CD4+T cells in patients with non-small cell lung cancer(NSCLC)with different pleural effusion severity and prognosis.Methods:A total of 104 NSCLC malignant pleural effusion patients admitted to Quanzhou First Hospital Affiliated to Fujian Medical University from January 2020 to December 2022 were selected as research subjects,and divided into three groups based on amount of pleural effusion,including small amount of pleural effusion group(35 cases),moderate amount of pleural effusion group(42 cases)and large amount of pleural effusion group(27 cases).According to actual development and prognosis of patient's disease,they were divided into good prognosis group(29 cases without recurrence and metastasis)and poor prognosis group(75 cases with recurrence and metastasis).Another 60 patients with benign pleural effusion due to pneumonia who were treated in Quanzhou First Hospital Affiliated to Fujian Medical University at same time were selected as control group.MEG3 expression in pleural effusion of two groups was detected by real-time fluorescent quantita-tive PCR,and peripheral venous blood of subjects was collected.Th17 cell and CD4+T cell ratios of peripheral blood were detected by flow cytometry,and Th17/CD4+T was calculated.lncRNA MEG3 and peripheral blood Th17 and CD4+T levels in each group of patients compared.Logistic regression analysis was used to analyze pleural effusion and prognostic factors in NSCLC.Results:lncRNA MEG3 expression and CD4+T percentage in pleural effusion in NSCLC group were lower than control group,while Th17 percentage and Th17/CD4+T were higher than control group(P<0.05).lncRNA MEG3 expression and CD4+T percentage in large pleural effusion group were lower than small and moderate pleural effusion groups.lncRNA MEG3 expression and CD4+T percentage in modarate pleural effusion group were lower than small pleural effusion group,while Th17 percentage and Th17/CD4+T in large pleural effusion group were higher than small and moderate pleural effusion groups.Th17/CD4+T was higher in small amount pleural effusion group(P<0.05).lncRNA MEG3 expression and CD4+T percentage in poor prognosis group were lower than those in good prognosis group,while Th17 percentage and Th17/CD4+T were higher than good prognosis group(P<0.05).Logistic regression analysis showed that lncRNA MEG3 was a protective factor for NSCLC pleural effusion,and Th17/CD4+T was a risk factor(P<0.05),lncRNA MEG3 was a protective factor of NSCLC prognosis,and Th17/CD4+T was a risk factor(P<0.05).Conclusion:lncRNA MEG3 expression and Th17/CD4+T in NSCLC patients with different pleural effusion severity and prognosis is not same.lncRNA MEG3 is a risk factor for NSCLC pleural effusion and prognosis,while Th17/CD4+T is a risk factor,which can be used as an effective biomarker for pleural effusion severity and progno-sis diagnosis.