Oxidative stress occurs as a result of reactive oxygen species (ROS) overproduction. The content of carbonyl groups (CG), malonyldialdehyde, 4-hydroxynonenal (MDA, 4-HNE) represent markers of protein and lipid peroxidation processes, respectively. The aim of the present study was to determine CG and MDA/4-HNE in the serum of 30 hemodialyzed patients (-HD; 13 men, 17 women of mean age 47.7 +/- 15.3 years) before and after a hemodialysis session, of 20 transplant patients (TX; 10 men, 10 women of mean age 40.7 +/- 11.3 years) before and after the procedure (RT), and of a control group (n = 47; including 30 women, 17 men of mean age 38.7 +/- 14.0 years). The CG content was evaluated using the 2,4-dinitrophenylhydrazine assay and MDA/4-HNE by the Oxis Bioxytech colorimetric method. Among the HD group the concentrations of MDA/4-HNE and CG were higher than control subjects (P <.05). In the HD group CG concentrations before and after dialysis session were similar while MDA/4-HNE concentrations were higher before the dialysis session (P <.01). One day after RT, MDA/4-HNE and CG concentrations had increased but at 7 days they had decreased and the CG level was increased. A high production of ROS can be assumed in dialysis patients. MDA/4-HNE concentrations, however, decreased after the dialysis treatment, because as low-weight molecules they diffused across the dialysis filter. On the first day after RT a high intensity of lipid and protein peroxidation was observed. During the first week after RT, accumulation of protein peroxidation products was observed but simultaneously lipid peroxidation product concentrations decreased due to quick metabolism. The intensity of lipooxidation during first day after RT seems to be dependent upon the ischemia time.