Selenium (Se) is a trace element which incorporates into the selenoenzyme glutathion peroxidase. Cataractogenesis may be caused either by the excess or deficiency of this trace element. More recently, its potential of becoming a possible environmental pollutant has been emphasized. In an attempt to reveal the relationship of this element with cataractogenesis, we detected its level in 48 serum, 36 lens and 9 aqueous humour samples of 48 patients with senile cataract, comparing the results with appropriate controls. Selenium levels (mean +/- SD) of cataractous patients were found to be 0.28 +/- 0.04 mu g/ml (CI: 0.27 to 0.29 mu g/ml) in sera (controls: 0.32 +/- 0.04 mu g/ml; CI: 0.30 to 0.34 mu g/ml, p < 0.0001), 5.43 +/- 3.07 mu g/g dry weight (CI: 4.43 to 6.43 mu g/g dry weight) in lens (controls: 4.43 +/- 2.53 mu g/g dry weight; CI: 2.78 to 6.08 mu g/g dry weight; p = 0.374) and 0.19 +/- 0.06 mu g/ml (CI: 0.15 to 0.23 mu g/ml) in aqueous humour samples (controls: 0.31 +/- 0.12 mu g/ml; CI: 0.24 to 0.38 mu g/ml, p = 0.02). When patient subgroups were analyzed, serum Se levels were found to be 0.28 +/- 0.05 mu g/ml (CI: 0.26 to 0.30 mu g/ml) in the nuclear cataract and 0.28 +/- 0.02 mu g/ml (CI: 0.27 to 0.30 mu g/ml) in the cortical cataract. Lens Se levels, on the other hand, were detected as 5.91 +/- 3.56 mu g/g dry weight (CI: 4.49 to 7.33 mu g/g dry weight) in the nuclear cataract and 4.47 +/- 1.40 mu g/g dry weight (CI: 3.68 to 5.26 mu g/g dry weight) in the cortical cataract. It is anticipated that decreased Se in aqueous humour and sera of patients with senile cataract may reflect defective antioxidative defense systems which may lead to the formation of cataract.