Along with the onset of severe kwashiorkor symptoms, a 20-month-old child showed biochemical signs of markedly increased lipid peroxidation, with a decrease of plasma antioxidants and decreased proportions of polyunsaturated fatty acids in plasma and red cell phospholipids. Additionally, plasma concentrations of the lipid peroxidation products malondialdehyde and hexanal, as well as the urinary excretion of leukotriene E4, were found to be increased. All biochemical alterations normalized along with subsequent clinical improvement. These findings suggest that the extent of lipid peroxidation is strongly related to the severity of the kwashiorkor syndrome.