Prostatitis is the third most important condition of the prostate. Problems in the diagnosis and classification of this condition, however, have hindered epidemiologic research. Consequently, our understanding of the incidence of prostatitis and factors that increase its probability is limited. Recent studies suggest that the prevalence of prostatitis is 5% to 9% among unselected men in the community. Based on estimates from national data, approximately 2 million men in the United States seek treatment for prostatitis each year. A number of recent studies suggest that genetic, behavioral, and environmental factors; age; inflammatory mediators; and oxidative stress may be risk factors for prostatitis. However, findings from most of these studies may at best be considered preliminary because of problems with small sample sizes or lack of generalizability. Nonetheless, these studies suggest hypotheses that should be tested in well-designed population-based studies.