To test the hypothesis that kwashiorkor is associated with increased oxidative stress, urinary concentrations of 2 oxidized amino acids, o,o '-dityrosine and ortho -tyrosine, were measured by gas chromatography-mass spectrometry. Children with kwashiorkor, with or without infection, had a 3- to 7-fold increase in urinary o,o '-dityrosine and a 1.5- to 2-fold increase in ortho -tyrosine when compared with well-nourished children. This observation raises the possibility that oxidative damage to proteins and other biologic targets plays a role in the clinical manifestations of kwashiorkor.