ABSTRACT: BACKGROUND: Sexually transmitted infections (STIs) are hypothesized to play a role in the development of prostate cancer, perhaps due to inflammation-induced oncogenesis. We assessed in a nationally representative population of middle-aged men whether sexual behavior indicators for an increased risk of genital infection were associated with serum prostate-specific antigen (PSA) concentration, a marker of prostatic disease and inflammation. RESULTS: The percentage of men between the ages of 40 and 59 with a PSA >/= 4.0 ng/ml was 2.6% (95% confidence interval [CI], 1.8% - 3.8%). The percentage of men between the ages of 40 and 59 self-reporting a past diagnosis of genital warts or genital herpes, or a recent diagnosis of gonorrhea or chlamydia is estimated to be 7.3% (95% CI, 6.2% - 8.6%). Men self-reporting that they had had sex without using a condom in the past month had a lower PSA concentration and higher %fPSA than those who did not. There were no associations between any of the other sexual activity or laboratory measures and PSA or %fPSA. CONCLUSION: In this nationally representative sample of middle-aged American men, we did not find consistent evidence for an association between sexual behavior or a history of STIs and PSA levels. Therefore, sexual factors are unlikely to lead to falsely elevated PSA tests in this population. We cannot rule out the role of these factors in causing false positive PSA tests in subgroups of the population that have a higher prevalence of high-risk sexual behavior, and more protracted or recent exposures to these agents.