Converging data from epidemiological and biological research implicate insulin-like growth factor (IGF) physiology in the regulation of prostate epithelial cell proliferation and in the pathophysiology of prostate cancer. This review (1) outlines elements of IGF physiology, (2) reviews recent evidence that circulating IGF-I level is related to risk of prostate cancer, (3) provides a hypothesis concerning the biological basis for the relationship between IGF-I level and risk of prostate cancer, (4) discusses IGF-I physiology in the context of neoplastic progression of prostate cancer, and (5) discusses clinical implications of these lines of research with respect to prevention and treatment.