['To examine plasma microRNA-21 (miR-21) level in patients with non-small cell lung cancer (NSCLC) and its potential correlation with chemotherapeutic response.', '77 NSCLC patients and 36 age and sex-matched healthy controls were included. Plasma miR-21 concentration was examined using a quantitative real-time reverse transcription polymerase chain reaction assay (qRT-PCR). Potential correlation between plasma mir-21 concentrations with chemotherapeutic responses was analyzed in 35 patients with advanced NSCLC (stages IIIB and IV).', 'Plasma miR-21 was significantly higher in NSCLC patients relative to the healthy controls (P<0.0001). As a biomarker, plasma mir-21 had a receiver operating characteristic (ROC) curve area of 0.729 with 61.04% sensitivity and 83.33% specificity. Chemotherapeutic response in the 35 patients with advanced NSCLC (stages IIIB and IV) included partial response (PR) (n=11), stable disease and progression disease (SD+PD) (n=24). The overall response rate (CR+PR) was 31.4%. Plasma miR-21 in patients who achieved PR was significantly lower than those who did not respond (SD+PD) (P=0.0487), and comparable to that of the healthy controls (P=0.2744).'] Is reduction of Plasma MicroRNA-21 Associated with Chemotherapeutic Response in Patients with Non-small Cell Lung Cancer?