Abstract MicroRNAs (miRNA) are small (21-25 nucleotides) non-coding regulatory RNAs that control protein expression at the transcriptional level through various mechanisms. Previous studies have shown that circulating miRNA can be utilized as a tool to gain a better understanding of both benign and malignant tumor conditions. Their altered expression has been linked with several cancers including lung cancer. Lung cancer is the leading cause of death worldwide with approximately 85% of diagnoses being attributed to non-small cell lung cancer (NSCLC). The goal of this study was to determine if the presence of circulating miRNAs from NSCLC patients can serve as non-invasive diagnostic biomarkers of disease and therapy monitoring. We initially evaluated the expression of 16 NSCLC-specific miRNAs (miR-146b, miR-155, miR-205, miR-21, miR-221, miR-199a-5p, miR-222, miR-320, miR-223, miR-25, miR-16, miR-191, miR-20a, miR-24, miR-145, and miR-152) in 46 NSCLC serum samples ranging from stage I to IV, including 20 adenocarcinoma and 26 squamous cell carcinoma samples. Eighteen healthy controls were used for comparison. RNA was extracted from serum using the single-step Trizol method and real-time PCR (Taqman). Of the 16 miRNAs tested, six miRNAs were significantly upregulated in the NSCLC when compared to the normals (p<0.0001 with AUROCs of 0.80 to 0.93). Top Scoring Pair analyses identified the ratio of miR-21 to miR-221 as the most robust predictor of NSCLC status compared to healthy subjects with an AUROC of 0.95 (95% CI; 0.91-0.99, p < 0.001) and 17% error rate (leave-one-out cross validation). This miRNA pair was then validated in an independent set of 19 NSCLC and 26 normal serum samples with overall accuracy of 80% (89% and 73% for NSCLC and normal, respectively) and AUC of 0.88 (95%CI: 0.78-0.98). These findings demonstrate the potential of using miR-21 and miR-221 in detecting NSCLC from the serum. Furthermore, these miRNAs could lead to the development of non-invasive biomarkers for early diagnosis, prognosis, and therapy monitoring in NSCLC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3015.