Prostate cancer is a major cause of cancer death among males, yet little is known about its etiology. We hypothesize that Vitamin (Hormone) D deficiency may underlie the major risks for prostate cancer, including age, Black race, and northern latitudes. These factors all are associated with decreased synthesis of Vitamin D. Mortality rates from prostate cancer in the US are inversely correlated with ultraviolet radiation, the principal source of Vitamin D. This hypothesis is consistent with known antitumor properties of Vitamin D, and may suggest new avenues for research in prostate cancer.