Lipid peroxidation products, both lipid hydroperoxides and thiobarbituric acid reactive substances (TBARS) were determined in the plasma of 31 uremic patients treated with maintenance hemodialysis. Whereas patients had significantly elevated TBARS compared to 93 healthy controls (4.25 ± 1.53 vs. 1.66 ± 0.50 μmol/l; p 0.01). However, when the TBARS were corrected for hemoconcentration by relating TBARS to the plasma cholesterol concentrations a statistically significant decrease of TBARS was observed (1.02 ± 0.63 μmol TBARS/mmol cholesterol vs. 0.84 ± 0.60 μmol TBARS/mmol cholesterol; p < 0.01) after 240 min of hemodialysis. There was no evidence for the formation of plasma lipid hydroperoxides in the extracorporeal circulation. It is therefore suggested that elevated TBARS in chronic renal failure are not caused by the dialysis