Background Pro‐inflammatory interleukin (IL)‐15 plays a major role in host defense and chronic inflammation by stimulating T‐lymphocyte recruitment and growth. Expression of IL‐15 and IL‐15 receptor (IL‐15R) in human prostate was examined. Methods Normal and benign hyperplastic (BPH) prostate specimens (n = 23) were analyzed for IL‐15 and IL‐15Rα‐chain expression by immunohistochemistry and Real‐Time‐PCR/RT‐PCR. Regulation of prostatic stromal cell (PSC) IL‐15 mRNA and effect of IL‐15 on prostatic cell growth were analysed in vitro. Results In normal prostate, anti‐IL‐15 and anti‐IL‐15Rα‐chain reactivity were restricted to smooth muscle and stromal cells. However, in BPH, in addition epithelial cells frequently exhibited discrete anti‐IL‐15R and often intense, membranous anti‐IL‐15 reactivity. IL‐15/IL‐15R mRNA were detected in all prostatic cells types. In BPH tissues, IL‐15 mRNA content was variable (15‐fold). IL‐15 mRNA synthesis of PSC was significantly up‐regulated by IFN‐γ. Furthermore IL‐15 strongly stimulated the growth of BPH‐T‐lymphocytes and weakly that of carcinoma cell lines, but not of stromal cells. Conclusions Overexpression of IL‐15 and IL‐15Rα‐chain in BPH and massive proliferation of BPH‐T‐lymphocytes induced by IL‐15 suggest a role for IL‐15 in prostatic inflammation. Since IFN‐γ, a T‐lymphocyte product, stimulates prostatic IL‐15 production; chronic inflammation might be triggered by this paracrine loop. Prostate 49:251–262, 2001. © 2001 Wiley‐Liss, Inc.