The two most important demographic characteristics of prostate cancer in Los Angeles are the high rates among blacks, which are two times those among whites and four times those among Asians, and the rapid increase in rates with age after age 40. Despite the high rates among blacks, a birth cohort analysis indicates that mortality rates among black men born after 1900 have decreased. In this report, epidemiologic and experimental evidence supporting each of three etiologic hypotheses--industrial exposure to cadmium, sexual transmission by an infectious agent, and endocrine factors--are reviewed. Evidence from descriptive data in Los Angeles suggests that only a small portion of cases might be attributable to industrial exposures. In a cohort study of Catholic priests, we found no deficit of prostate cancer mortality, strong evidence against sexual transmission of the disease. Experimental evidence and a limited amount of human data support an endocrine hypothesis. Preliminary results of a case-control study of prostate cancer are presented, but these results are unable to distinguish among these hypotheses further. This study finds a substantial protective effect of vasectomy, an event that is accompanied by reduced prostatic function and size, but this result is thus far statistically insignificant.