Lipid peroxidation, as measured by the thiobarbituric acid test, has been reported to have increased in hemodialysis (HD) patients, even though the test has low specificity in vivo. Conjugated diene fatty acid (CDFA) hydroperoxides are formed during lipid peroxidation, but not all conjugated dienes (CD) detected in humans originate from lipid peroxidation: octadeca-9,11-dienoic acid, a nonhydroperoxide CD derivative of linoleic acid (CDLA), has a dietary origin. We evaluated CDFA hydroperoxides, CDLA and linoleic acid, using high-performance liquid chromatography, in lipids extracted from plasma, adipose tissue and RBC membranes obtained from 25 patients treated with HD, 16 patients treated with hemodiafiltration (HDF) and 29 controls. No differences in the levels of CDFA hydroperoxides and linoleic acid were seen in any of the groups. Concentrations of CDLA were found to be significantly high in the adipose tissue and low in the RBC membranes of HD patients. HDF-treated patients showed the same results as HD patients. No direct evidence of increased lipid peroxidation was found in HD patients. This does not exclude the possibility that lipid peroxidation is increased and escapes direct detection due to the body's homeostatic control eliminating the increased production of hydroperoxides. Both HD- and HDF-treated patients showed a significant change in CDLA concentrations, either in the adipose tissue, or in the RBC membranes. These dietary CD may be mistaken for markers of lipid peroxidation by conventional methodologies.