Chemoprevention is the administration of agents to prevent induction and inhibit or delay progression of cancers. For prostate, as for other cancer targets, successful chemopreventive strategies require well-characterized agents, suitable cohorts, and reliable intermediate biomarkers of cancer for evaluating chemopreventive efficacy. Agent requirements are experimental or epidemiological data showing chemopreventive efficacy, safety on chronic administration, and a mechanistic rationale for the observed chemopreventive activity. On this basis, promising chemopreventive drugs in prostate include retinoids, antiandrogens, antiestrogens, steroid aromatase inhibitors, 5alpha-reductase inhibitors, vitamins D and E, selenium, lycopene, and 2-difluoromethylornithine. Phase II trials are critical for evaluating chemopreventive efficacy. Cohorts in these trials should be suitable for measuring the chemopreventive activity of the agent and the intermediate biomarkers chosen as endpoints. Many cohorts proposed for phase II trials are patients with previous cancers or premalignant lesions. For such patients, trials should be conducted within the context of standard treatment. Two cohorts currently used in phase II prostate cancer chemoprevention trials are patients with PIN and patients scheduled for prostate cancer surgery. Biomarkers should fit expected biological mechanisms, be assayed reliably and quantitatively, measured easily, and correlate to decreased cancer incidence. Protocols for adequately sampling tissue are essential. Changes in PIN provide prostate biomarkers with the ability to be quantified and a high correlation to cancer. PIN measurements include nuclear polymorphism, nucleolar size and number of nucleoli/nuclei, and DNA ploidy. Other potentially useful biomarkers are associated with cellular proliferation kinetics (e.g. PCNA and apoptosis), differentiation (e.g. blood group antigens, vimentin), genetic damage (e.g. LOH on chromosome 8), signal transduction (e.g. TGFalpha, TGFbeta, IGF-I, c-erbB-2 expression), angiogenesis, and biochemical changes (e.g. PSA levels).