A new colorimetric method for quantitative analysis of serum lipid peroxide, free of interference from sialic acids, has been developed. We have used the thiobarbituric acid dissolved in sodium sulfate solution and both liberation of lipid peroxide and color reaction have been performed simultaneously by heating serum protein precipitate with this reagent in a weak acid solution. The new method is specific and facilitates the precise measurements of serum lipid peroxide. The average values determined by the new method increased slightly with age in healthy subjects. In patients with sequelae of cerebrovascular disorders, serum lipid peroxide values were higher than in healthy controls. These results may demonstrate the important role of lipid peroxide in aging and cerebrovascular disorders.