Neuropilin-1 (NRP-1), a co-receptor for VEGF165, is overexpressed in various prostate cancer cell lines and in advanced prostate tumors. However, distribution of the NRP-1 in prostate tumors has not yet been evaluated. Using immunohistochemical analysis, we evaluated 21 archival prostate tumors and 5 benign glands for the expression of NRP-1. In addition, we utilized a quantitative RT-PCR method to examine mRNA expression in 9 additional prostate tumors obtained from radical prostatectomy specimens and compared this expression to the adjacent normal tissue. The RT-PCR analyses demonstrated overexpression of NRP-1 mRNA in malignant tissue samples by 10.0-fold as compared to adjacent normal tissue. By immunohistochemistry, NRP-1 protein was undetected or minimally detected in the epithelial tumor cells. However, NRP-1 immuno-reaction was detected in the surrounding tumor stroma. Variable immuno-reaction for NRP-1 was also seen in the adjacent normal tumor stroma and the stroma of the benign prostate samples. These observations suggest that neuropilin-1 is expressed in the prostatic stromal cells, not epithelial tumor cells, and this expression is significantly increased in the malignant phenotype.