Abstract: Objective To discuss the clinical value of DNA damage detection of peripheral blood mononuclear cells in the diagnosis of multiple primary lung cancer and metastatic carcinoma. Methods By micro-whole blood single cell gel electrophoresis assay (comet experiments), we detected DNA damage rate and damage degree of peripheral blood mononuclear cells of 28 patients with pulmonary metastasis carcinoma, 30 patients with multiple primary lung cancer, and 29 patients with primary lung cancer. Results DNA damage rate of peripheral blood mononuclear cell in pulmonary metastatic carcinoma group was 6.29%(176/2800), which was significantly higher than 3.77%(113/3000) in multiple primary lung cancer group(χ²=18.883, P=0.000); moreover, DNA damage rate of peripheral blood mononuclear cell in multiple primary lung cancer group was also significantly higher than 2.66% (77/2900) in primary lung cancer group(χ²= 5.494, P= 0.019). DNA damage tail length, tail moment and olive moment of patients in pulmonary metastatic carcinoma group(44.19±13.48)μm, (4.53±1.66), (4.64±1.50) were significantly higher than those in multiple primary lung cancer group(34.85±8.40)μm,(3.30±1.04) and (3.54±1.03)(P=0.000); moreover,DNA damage tail length, tail moment and olive moment of patients with multiple primary lung cancer group was significantly higher than those in primary lung cancer group(26.25±7.64)μm, (2.20±0.98) and (2.37±0.78)(P=0.000). Conclusion The detection of DNA damage in peripheral blood mononuclear cells is helpful to diagnose the difference between multiple primary lung cancer and pulmonary metastatic carcinoma.