Abstract: Objective : To investigate the relationship of neoadjuvant chemotherapy and bone marrow micrometastasis of stage II non-small cell lung cancer (NSCLC). Methods : CK19 mRNA expression was detected by RT-PCR in rib marrow sample from 36 patients after neoadjuvant chemotherapy during operation,with sample of 35 patients without neoadjuvant chemotherapy during operation as Control,all patients were stage III NSCLC. Results : The positive rates of CK19 mRNA were 22.2% (8/36)and 45.7% (16/35) in the neoadjuvant group and the control group respectively (P =0.026). There was a negative relationship existing between the expression of CK19 mRNA and the sensitivity of chemotherapy (P=0.026), r_s =-0.3720 Cox's model showed that the sensitivity of chemotherapy and the expression of CK19 mRNA had separate and significant impacts on prognosis in neoadjuvant group. Conclusions : Neoadjuvant chemotherapy could reduce the positive rate of micro-metastasis. Detecting the expression of CK19 may lead to an early diagnosis and treatment of patients with subclinical metastasis in bone marrow.