Emerging evidence shows that microRNAs (miR) are involved in the pathogenesis of a variety of cancers, including prostate carcinoma (PCa). Little information is available regarding miR expression levels in lymph node metastasis of prostate cancer or the potential of miRs as prognostic markers in this disease. Therefore, we analyzed the global expression of miRs in benign, hyperplastic prostate tissue (BPH), primary PCa of a high risk group of PCa patients, and corresponding metastatic tissues by microarray analysis. Consistent with the proposal that some miRs are oncomirs, we found aberrant expression of several miRs, including the downregulation of miR-221, in PCa metastasis. Downregulation of miR-221 was negatively correlated with the expression of the proto-oncogen c-kit in primary carcinoma. In a large study cohort, the prostate-specific oncomir miR-221 was progressively downregulated in aggressive forms of PCa. Downregulation of miR-221 was associated with clinicopathological parameters, including the Gleason score and the clinical recurrence during follow up. Kaplan-Meier estimates and Cox proportional hazard models showed that miR-221 downregulation was linked to tumor progression and recurrence in a high risk prostate cancer cohort. Our results showed that progressive miR-221 downregulation hallmarks metastasis and presents a novel prognostic marker in high risk PCa. This suggests that miR-221 has potential as a diagnostic marker and therapeutic target in PCa.