Urine from patients with calcium-4-(2,4-dihydroxy-3,3-dimethylbutyramido) butyrate hemihydrate (hopantenate) therapy during episodes of Reye's-like syndrome was found to contain a number of unusual dicarboxylic acids in high concentrations; odd- and even-numbered medium-chain dicarboxylic acids, alpha-hydroxydicarboxylic acids and beta-hydroxydicarboxylic acids. The abnormal excretion of dicarboxylic acids, alpha- and beta-hydroxydicarboxylic acids disappeared after discontinuance of hopantenate therapy. Besides the excretion of 2-hydroxydecandedioic acid, which has been previously described in Zellweger syndrome or neonatal adrenoleukodystrophy, a series of alpha-hydroxydicarboxylic acids was detected and identified. In this paper, we have characterized some new compounds by gas chromatography/mass spectrometry: 2-hydroxydodecanedioic acid, 2-hydroxydodecenedioic acid, 2-hydroxytetradecanedioic acid, 2-hydroxytetradecenedioic acid and 2-hydroxyoctanedioic acid.