Novel molecular targets are being searched to aid in prostate cancer diagnosis and therapy. Recently, ZFP91 zinc finger protein has been found to be upregulated in prostate cancer cell lines. It is a potentially important oncogenic protein; however only limited data regarding its biological function and expression patterns are available. To date, ZFP91 has been shown to be a key factor in activation of noncanonical NF-κB signaling pathway as well as to be involved in HIF-1α signaling in cancer cells. The present study aimed to characterize ZFP91 expression in prostate cancer specimens. Furthermore, since our earlier reports showed discrepancies between ZFP91 mRNA and protein levels, we studied this interrelationship in LNCaP and PC-3 prostate cancer cell lines using siRNA mediated knockdown. QPCR analysis revealed marked upregulation of ZFP91 mRNA in the majority of prostate cancer specimens. Transfection of prostate cancer cells with ZFP91 siRNA resulted in a 10-fold decrease in mRNA levels. On a protein level, however, no inhibitory effect was observed over the time of the cell culture. We conclude that ZFP91 is overexpressed in prostate cancer and that potential accumulation of the ZFP91 protein in studied cells may be of importance in prostate cancer biology.