The selenium concentrations in serum and erythrocytes and the erythrocyte glutathione peroxidase activity were determined in 15 boys with the Duchenne type and in 5 boys with the Becker type of X-linked muscular dystrophy before and during long-term selenium and alpha-tocopherol supplementation and compared with values in unsupplemented controls. The purpose of the treatment was to improve the muscular strength. Twelve of the 20 patients had pretreatment levels of selenium in serum that were within the 95% confidence limit of the unsupplemented control children. The values in 2 patients, both with the Duchenne type of muscular dystrophy, fell below this level. Selenium supplementation in a daily dose of 6 micrograms/kg/day for 6 months caused a substantial rise in both serum and erythrocyte selenium, suggesting suboptimal pretreatment body contents of selenium. The greatest increases in both serum and erythrocyte selenium were observed in subjects with initially low selenium levels. Only in 4 of the 20 patients did the selenium supplementation result in a significant rise in erythrocyte glutathione peroxidase activity. As no sure improvement was noted in muscular strength during this treatment period, the Se dose was increased to 20 micrograms/kg/day. This resulted in a further rise in both serum and erythrocyte selenium, but not in erythrocyte glutathione peroxidase activity.