Erythrocytes are target cells for peroxidative damage. Abnormal susceptibility of erythrocyte lipids to peroxidation is believed to reflect a similar abnormality in other organs and tissues. The changes in erythrocyte lipid peroxidation [measured by malonyldialdehyde (MDA) concentration] and erythrocyte membrane cholesterol (EMC) and their correlation with plasma lipid changes were studied in 36 children with steroid-responsive minimal change nephrotic syndrome (MCNS) (16 in relapse, 20 in remission) and 30 matched healthy controls. Erythrocyte MDA levels were significantly higher in relapse [126.3+/-40.6 nmol/g hemoglobin (Hb)] com pared with remission (101.2+/-21.3 nmol/g Hb, P<0.02) and in controls (95.4+/-20.4 nmol/g Hb, P<0.001). Plasma MDA levels in relapse were also higher than in remission (4.26+/-1.19 nmol/ml vs. 3.16+/-1.18 nmol/ml, P<0.01), and in controls (2.49+/-0.86 nmol/ml, P<0.001). The EMC content changed significantly during remission (1.22+/-0.15 mg/10(10) cells in relapse, 1.09+/-0.19 mg/10(10) cells in remission, P<0.04). These results show an increased sensitivity of red cells to lipid peroxidation in patients with steroid-sensitive nephrotic syndrome without the development of renal failure and anemia. Lipid peroxidation of plasma lipids and erythrocyte membrane may be a primary phenomenon, but this should be confirmed by investigation of peroxidation of renal lipids.