Plasma selenium and glutathione peroxidase in erythrocytes were analysed in a case-control study encompassing 164 cases with prostate cancer and 152 controls with benign prostate hyperplasia. Plasma selenium levels were divided into three groups; I > 1.17, II 1.00-1.17 and III < 1.00 mumol/l. For the 124 cases with no supplementary intake of selenium pills, the mean plasma selenium level was 0.99 (range 0.27-1.47) and for the corresponding 121 controls 1.08 (range 0.52-1.50) mumol/l, a difference which was significant (P = 0.0007). The three categories of selenium levels had odds ratios (OR) of 0.3 and a 95% confidence interval (CI) of 0.1-0.7 for group I, an OR of 0.6 and a CI of 0.3-1.1 for group II, and group III was used as the reference entity. No significant differences in levels of glutathione peroxidase in erythrocytes were found between cases and controls.