PURPOSE: Emerging evidences indicate that dysregulated microRNAs are implicated in cancer tumorigenesis and progression. MicroRNA-9 (miR-9) has various expression patterns in diverse human cancers. However, its clinical significance in human non-small cell lung cancer has not yet been elucidated. In the present study, we detected the expression of miR-9 in non-small cell lung cancer and adjacent noncancerous tissues and explored its relationships with clinicopathological characteristics and prognosis. METHODS: Expression levels of miR-9 in 116 pairs of non-small cell lung cancer and adjacent normal tissues were detected by real-time quantitative RT-PCR assay. To determine its prognostic value, overall survival (OS) and progression-free survival (PFS) were evaluated using the Kaplan-Meier method. Univariate and multivariate analysis were performed using the Cox proportional hazard analysis. RESULTS: MiR-9 expression in non-small cell lung cancer tissues was significantly higher than that in adjacent normal tissues (p = 0.001), and its up-regulation was significantly correlated to advanced tumor-node-metastasis (TNM) stage (p < 0.001), tumor size (p = 0.013), and lymph node metastasis (p = 0.001). Furthermore, Kaplan-Meier analysis demonstrated that high miR-9 expression clearly predicted poorer PFS (p < 0.001) and OS (p < 0.001). In the multivariate analysis, increased miR-9 expression was an independent prognostic factor for both PFS (p = 0.002) and OS (p = 0.013). CONCLUSIONS: MiR-9 was up-regulated in non-small cell lung cancer tissues and correlated with adverse clinical features and unfavorable survival, indicating that miR-9 might be involved in non-small lung cancer progression and could serve as a promising biomarker for further risk stratification in the treatment of this cancer.