Lipid peroxidation products, lipid hydroperoxide and malonaldehyde were measured in the serum of patients on maintenance hemodialysis and compared with values obtained from a group of healthy controls. Dialysis patients had significantly elevated levels of lipid peroxidation products (12.9 ± 1.9 vs. 9.3 ± 1.3 nmol/ml, mean ± SD, patients vs. controls, p < 0.0001). In contrast, the serum transferrin level was significantly lower in the dialysis patients (217 ± 71 vs. 290 ± 50 mg/dl, patients vs. controls, p < 0.0002). Compared to normal controls HDL cholesterol was lower (40 ± 11 vs. 53 ± 8, p < 0.0001) and triglycerides were higher (206 ± 103 vs. 142 ± 64, p < 0.007) in the patient group. The patient group was then divided on the basis of using polysulfone or cupraphane dialyzers. Patients using polysulfone dialyzers had lower levels of 2-thiobarbituric acid-reactive substances (12.3 + 0.9 vs. 13.6 ± 2.1, polysulfone vs. cupraphane, p = 0.05), and higher transferrin concentrations (248 ± 84 vs. 194 ± 37, p = 0.03). Thus, in patients on maintenance hemodialysis there was evidence for accelerated lipid peroxidation. This abnormality was more marked in patients using traditional cupraphane dialyzers. The mechanism for this observation rema