Recent studies have shown that diabetic men have a lower risk of prostate cancer and that this association may be related to time since diagnosis. The authors examined the association between diabetes and prostate-specific antigen (PSA) levels, controlling for potential confounders, in a nationally representative cross-sectional survey of the US population (National Health and Nutrition Examination Survey 2001-2002). Diabetes classification was self-reported, and undiagnosed diabetes was determined with fasting plasma glucose measurements. Controlling for age, men with self-reported diabetes had a 21.6% lower geometric mean PSA level than men without diabetes. The difference increased with years since diagnosis (>10 years: 27.5% lower geometric mean PSA level). Overweight men who had had diabetes for more than 10 years had a predicted geometric mean PSA level 40.8% lower than that of nondiabetic, normal-weight men. These results are consistent with the hypothesis that long-term diabetes is associated with a lower risk of prostate cancer. The mechanism of this association may involve the regulation of PSA by androgens, although the authors are unable to confirm this assertion. Better understanding of the determinants of PSA level is needed to make the distinction between factors affecting the PSA test's accuracy and those altering the risk of prostate cancer.