Clinical trial and observational studies on selenium and the risk of prostate cancer listed by mean or median selenium concentrations. ORs and 95% CIs were obtained in a comparison of the highest with the lowest quantile of selenium. ◆, randomized clinical trials with selenium concentrations obtained from serum or plasma; ▲, nested case-control study (CCS) and case cohort study with selenium concentrations obtained from serum or plasma; ●, population-based CCS with selenium concentrations obtained from serum or plasma; △, nested CCS and case cohort study with selenium obtained from toenails; ○, population-based CCS with selenium obtained from toenails. Observational studies were excluded if they used a questionnaire-based assessment of selenium intake, included patients with benign prostatic hyperplasia as controls, or included < 15 prostate cancer cases.