Objective To discuss the possibility of mucin-1 gene(MUC1 mRNA)in mediastinal lymphnodes(MLNs)and peripheral blood as a symbol of occult metastasis in patients with non-small cell lung cancer(NSCLC).Methods By reverse transcriptase-polymerase chain reaction(RT-PCR),MUC1 mRNA in peripheral blood and in 67 MLNs from 30 patients with benign pulmonary diseases(negative control group)and in 30 MLNs which had metastasis determined by routine pathologic examination(positive control group)was detected;MUC1 mRNA in peripheral blood in 158 patients with NSCLC and in 296 MLNs proven to be tumor-free by routine histopathologic examination from 97 NSCLC patients was detected(experimental group).Results MUC1 mRNA was not identified in peripheral blood in 30 patients with benign pulmonary diseases,but 49 specimens from 158 NSCLC patients were positive(positive rate 31.0%),there was significant difference between these two groups(P0.05),but the positive rate of MUC1 mRNA in stage Ⅲ and Ⅳ NSCLC was significantly higher than that in stage Ⅰ and Ⅱ(P