BACKGROUND: Prostatitis and sexually transmitted diseases (STDs) have been positively associated with prostate cancer in previous case-control studies. However, results from recent prospective studies have been inconclusive. METHODOGY/PRINCIPAL FINDINGS: We investigated the association between prostatitis, STDs, and prostate cancer among African American, Asian American, Latino, and White participants of the California Men’s Health Study. Our analysis included 68,675 men, who completed a detailed baseline questionnaire in 2002-2003. We identified 1,658 incident prostate cancer cases during the follow-up period to June 30, 2006. Cox proportional hazards models were used to estimate relative risks and 95% confidence intervals. Overall, men having a history of prostatitis had an increased risk of prostate cancer than men with no history (RR = 1.30; 95% CI: 1.10-1.54). Longer duration of prostatitis symptoms was also associated with an increased risk of prostate cancer (P trend = 0.003). In addition, among men screened for prostate cancer (1 or 2 PSA tests), a non-significant positive association was observed between prostatitis and prostate cancer (RR = 1.10; 95% CI: 0.75-1.63). STDs were not associated with overall prostate cancer risk. In racial/ethnic stratified analysis, Latinos reporting any STDs had an increased risk of disease than those with no STDs (RR = 1.43; 95% CI: 1.07-1.91). Interestingly, foreign-born Latinos displayed a larger risk associated with STDs (RR = 1.87; 95% CI: 1.16-3.02) than U.S. born Latinos (RR = 1.15; 95% CI: 0.76-3.02). CONCLUSION: In summary, results from this prospective study suggest that prostatitis and STDs may be involved in prostate cancer susceptibility. While we cannot rule out the possible influence of incidental detection, future studies are warranted to further investigate the role of infectious agents related to prostatitis and STDs in prostate cancer development.