To analyze differential gene expression of putative prostate tumor markers we compared the expression levels of more than 400 cancer-related genes using the cDNA array technique in a set of capsule-invasive prostate tumor and matched normal prostate tissue. The overexpression of Bax inhibitor-1 (BI-1) in prostate carcinoma and prostate cancer cell lines was confirmed by using Northern blot and Western blot analyses. Quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) on intact RNAs from 17 paired laser-captured microdissected epithelial tissue samples confirmed up-regulated BI-1 expression in 11 of 17 prostate tumors. In addition, it was demonstrated that BI-1 expression is down-regulated in stromal cells as compared to matched normal epithelial cells of the prostate. In situ hybridization experiments on prostate sections also revealed that BI-1 expression is mainly restricted to epithelial cells. Furthermore, quantitative RT-PCR on RNAs derived from five benign prostate hyperplasia (BPH) samples showed no significant difference in BI-1 expression as compared to normal epithelial prostate tissue. To determine the function of BI-1 in vitro, human PC-3, LNCaP, and DU-145 prostate carcinoma cells were transfected with small interfering double-strand RNA (siRNA) oligonucleotides against the BI-1 gene leading to a specific down-regulation of BI-1 expression. Furthermore, transfection of PC-3, LNCaP, and DU-145 cells with BI-1 sequence-specific siRNAs caused a significant increase in spontaneous apoptosis in all cell lines. Taken together, our results indicate that the human BI-1 gene contains the potential to serve as a prostate cancer expression marker and as a potential target for developing therapeutic strategies for prostate cancer.