AIM: To investigate the effect of CDA-G208A gene polymorphism on the efficacy and safety of gemcitabine in the first-line treatment of lung squamous cell carcinoma. METHODS: Sixty-five first-line treated patients with locally advanced or metastatic lung squamous cell carcinoma in The First Affiliated Hospital of Wannan Medical College hospital were screened. Group A included 31 pa-tients tested with the GG (wild homozygous) CDA-G208A gene, and group B included 34 patients without testing. All patients received gemcitabine plus platinum chemotherapy for at least 2 cycles. The efficacy and safety were evaluated following the RECIST 1.1 standard and the NCI-CTC 5.0 stan-dard, respectively. The primary study endpoint was progression-free survival (PFS), overall survival (OS) and the secondary study endpoints included objec-tive effective rate (ORR), disease control rate (DCR), adverse reactions, and influencing factors of PFS. RESULTS: The results showed that the DCR was 74.5% and 50% in group A and group B, respective-ly (P=0.045); mPFS was 6.1 months and 5.0 months in group A and group B, respectively (P=0.034); and the mOS was 13.3 months and 12.0 months in group A and group B, respectively, and there was no statistical difference (P=0.388). The number of cases of grade III-IV neutropenia in group A and group B was 2 and 10, respectively (P=0.017); grade III-IV neutropenia was an independent prog-nostic factor affecting patients with PFS (P=0.045); the group with unknown G208A gene status was more likely to develop grade III-IV neutrophils (P= 0.029). The AUC of CDA-G208A gene predicting neutropenia caused by gemcitabine chemotherapy was 0.756. CONCLUSION: Non-GG type of CDA-G208A gene can reduce the metabolic rate of gem-citabine in the body and cause neutropenia after chemotherapy. In severe cases, it can indirectly re-duce the clinical efficacy of gemcitabine. The detec-tion of CDA-G208A gene status before treatment can predict the neutropenia caused by gemcitabine chemotherapy.