For some time it has been claimed that trisomic cells are more sensitive to oxidative stress since there is an imbalance in hydrogen peroxide metabolism due to an increase in superoxide dismutase (SOD) catalytic activity. We designed the present study to assess activity levels of antioxidant enzymes [superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT) and glucose-6-phosphate-dehydrogenase (G6PDH)] in erythrocytes in 31 male adolescents with Down syndrome (mean age 16.3 +/- 1.1). An increase of 35.2%, 15.3% and 14.9% in the catalytic activity of SOD, GPx and G6PDH respectively was observed in male adolescents with Down syndrome compared to age-matched controls. For CAT, a slight increase of 6.0% was also found. It is concluded that our data are consistent with previous evidence of the existence of oxidative stress in individuals with Down syndrome as revealed by significantly enhanced activities of SOD and GPx. The most striking feature was that G6PDH, in contrast to CAT, presented a similar behaviour. Further studies are required to identify other antioxidant enzymes in red blood cells as well as in white blood cells in order to increase the range of potential bioindicators of oxidative stress.