The resistance of small cell lung cancer (SCLC) to anticancer drugs is a serious clinical problem often encountered during chemotherapy. Therefore, how to prevent this drug resistance need to be investigated. Multidrug resistance 1 (MDR1) gene and multidrug resistance-related protein (MRP) gene, two genes known to be associated with the development of drug resistance, are very common in SCLC. The purpose of this study was to evaluate retrospectively the relationship between chemotherapy responses to MDR1 gene encodes 170 kDa P-glycoprotein (Pgp) expression or MRP gene encodes 190 kDa MRP expression in SCLC. Before chemotherapy, multiple nonconsecutive sections of the bronchoscopy biopsy specimens of SCLC from 50 patients were analyzed immunohistochemically to detect Pgp and MRP expressions. Chemotherapy responses of the 50 patients were evaluated in the third month after completion of treatment by clinical and radiological methods. Of the 23 SCLC patients with poor response to chemotherapy, 11 had positive Pgp and MRP expressions, 2 had positive Pgp but negative MRP expressions, 6 had positive MRP but negative Pgp expressions, and 4 patients had negative Pgp and MRP expressions. All 27 SCLC patients with good response had negative Pgp and MRP expression. Immunohistochemical analyses of Pgp or MRP expression are potential tools for predicting patients' chemotherapy response in SCLC.