The accumulated data indicate that asthma is associated with reduced circulatory selenium (Se) status and lowered activity of the Se-dependent enzyme glutathione peroxidase (GSH-Px), which may have etiological implications, considering the important role of GSH-Px in the cellular elimination of hydroperoxides. The aim of the present double-blind study was to investigate whether Se supplementation in asthmatic patients may increase GSH-Px activity and possibly bring about clinical improvement. Twenty-four patients suffering from intrinsic asthma were selected and randomized into two groups, and after a preintervention period of 4 weeks, one group received a daily supplement of 100 micrograms sodium selenite for 14 weeks, whereas the other group received placebo. In the Se-supplemented group there were significant increases in serum Se and platelet GSH-Px activity after intervention, accompanied by a significant reduction in the irreversible platelet aggregation induced by 5 mumol/l ADP, while no significant changes in these parameters could be observed in the placebo group. Further, there was a significant clinical improvement in the Se-supplemented group, as compared with the placebo group, with regard to the assembled clinical evaluation made of each patient. This improvement could, however, not be validated by significant changes in the separate clinical parameters of lung function and airway hyperresponsiveness. The results are discussed in view of the role of GSH-Px in the cellular enzymatic oxidant defense system and as a modulator of arachidonic acid metabolism.