Human P-glycoprotein (Pgp) encoded by the MDR1 gene confers multidrug resistance to cancer cells. The clinical role of MDR1/Pgp in lung cancer is not fully understood. A total of 87 lung cancer surgical tissue samples, including previously untreated 84 non-small-cell (NSCLC) and three small-cell lung carcinoma (SCLC), were analyzed for levels of MDR1 mRNA determined by Northern blotting and compared with MDR1-positive cell lines. Fifteen percent (13/87) of the tumors were positive for the MDR1 gene, but the level was low in all samples except in one adenocarcinoma which expressed a high level of MDR1. The gene expression in these tumors did not relate with any pathologic factors such as histologic type, pathologic stage and tumor size. The SCLC and only one of the 14 MDR1-negative NSCLC responded to adjuvant chemotherapy after surgery. The present results indicate that the MDR1 gene is not associated in NSCLC with tumor progression and drug resistance.