Incidental foci of prostate cancer are found at autopsy in 30% of men in their third decade, and by their eighth decade more than 75% have histological evidence of cancer. This unprecedented cancer prevalence points to a ubiquitous causative agent or perhaps an interaction between multiple common carcinogenic cofactors. We propose that one of these carcinogens is Propionibacterium acnes. Several characteristics of prostate cancer suggest the involvement of an infectious agent and we provide evidence that P. acnes is an excellent candidate. We have cultured P. acnes from a substantial proportion of prostate glands containing cancer and shown a significant positive association with prostatic inflammation. P. acnes is well suited to cause persistent, low-grade infection involving a marked inflammatory response and the P. acnes subtypes most frequently associated with prostate cancer become highly prevalent in the urinary tract of males following puberty.