In the present study, the clinical application of a new functional test for vitamin E deficiency was evaluated. Erythrocytes from cholestatic children at risk for vitamin E deficiency and appropriate controls were incubated in vitro with hydrogen peroxide and the malondialdehyde generated and released into the supernatant quantitated. The results of these incubations were compared with fasting plasma vitamin E levels, the ratio of plasma vitamin E to plasma lipid levels, and, in some instances, hydrogen peroxide hemolysis tests. Malondialdehyde formation was less than 6% in controls and vitamin E-sufficient cholestatic children. However, cholestatic vitamin E-deficient children had a mean malondialdehyde formation of 41%. The results also suggest that for children less than 4 months of age, a ratio of plasma vitamin E to total plasma lipids less than 0.6 mg/g may be sufficient to provide protection from in vitro peroxidation. The authors suggest that this functional assay of vitamin E status be included in the evaluation of individuals with the potential for vitamin E deficiency.