Children with Down's syndrome suffer many diseases among which cardiovascular diseases, increased susceptibility to infections, leukemia, endocrine alterations, immune defects, nutritional disturbance and mental retardation have clinical relevance. It has been suggested that the pathogenesis of Down's syndrome involves reactive oxygen species arising from a mutation in gene encoding, which disproportionately elevates superoxide dismutase activity. Reactive oxygen species and total antioxidant capacity were evaluated using two new spectrophotometric methods in a selected group of 40 children with Down's syndrome and in 20 apparently healthy children used as controls. Reactive oxygen species were significantly higher (p <0.05) in children with Down's syndrome than in controls: 452 (+/- 72) U.Carr vs. 270 (+/- 66) U.Carr respectively. Total antioxidant capacity was significantly higher (p <0.05) in controls than in children with Down's syndrome: 380 (+/- 52) micromol hypochlorous acid (HCLO)/ml vs. 281 (+/- 33) micromol HCLO/ml, respectively. In fact, thiol groups (sulfhydryl) were significantly higher (p <0.05) in controls than in children with Down's syndrome: 644 (+/- 78) micromol/l vs. 462 (+/- 54) micromol/l, respectively Our data show how to simply measure chemical indices of oxidative status in serum samples from children with Down's syndrome. We determined the plasmatic activities of reactive oxygen metabolites and oxidative defense molecules. Accumulated macromolecular damage may be one of the causes of some of the abnormalities that are considered part of the syndrome. Therefore, children with Down's syndrome have to cope with a significant prooxidant environment. Oxidative stress causes alterations such as atherosclerosis, early aging, immunological default and neurologic disorders in Down's syndrome patients. The new test available for measuring reactive oxygen species in serum proved to be reliable and useful as an early marker of tissue damage.