Dialysis is associated with an impairment of antioxidant defense and an overproduction of oxidative stress markers. This study focuses on the comparison of plasma total antioxidant capacity (TAC) and lipid peroxidation products in patients on hemodialysis (HD) before and after treatment and in peritoneal dialysis (PD) patients. Plasma TAC, malonaldehyde (MDA), and 4-hydroxyalkenal concentrations were measured in 31 HD patients, in 24 PD patients, and in 17 normal controls (NC). It was found that before HD, TAC and MDA levels were higher than those in the NC (p < 0.001). After HD, these levels decreased significantly but were higher than in NC and lower than in PD patients (p < 0.001). The levels of 4-hydroxyalkenals were elevated in patients as compared with NC, but did not differ between HD and PD patients. The MDA concentrations correlated positively with the TAC in the patients. It is concluded that the oxidative status of patients on HD is similar to that of patients on PD and that the susceptibility to oxidative stress is strongly related to the levels of MDA produced in plasma.