Abstract The contributions of interleukin (IL)-17 to cancer remain unclear and somewhat controversial. We took a genetic approach to explore its role in prostate cancers by interbreeding IL-17 receptor C (IL-17RC)–deficient mice with mice that are conditionally mutant for PTEN, one established preclinical model for prostate cancer. Mice that were IL-17RC–deficient (IL-17RC−) displayed prostates that were smaller than mice that maintained IL-17RC expression (IL-17RC+). In addition, IL-17RC− mice developed a reduced number of invasive prostate adenocarcinomas with lower rates of cellular proliferation and higher apoptosis than IL-17RC+ mice. Moreover, the fibromuscular stroma surrounding prostatic glands was relatively thicker in IL-17RC− mice and was associated with decreased matrix metalloproteinase (Mmp)7 expression and increased Timp1, 2, and 4 expression, whereas administration of recombinant mouse IL-17 induced prostatic expression of Mmp7. Taken together, our results suggested that IL-17 promotes the formation and growth of prostate adenocarcinoma, and that an IL-17–MMP7 signaling axis is required for the transition of prostatic intraepithelial neoplasia to frank adenocarcinoma. Cancer Res; 72(10); 2589–99. ©2012 AACR.