The study was conducted on 80 patients suffering from chronic renal failure, who were admitted to the dialysis centers for peritoneal and hemodialysis therapy. They were divided as follows: 1. Group I: patients who ended the peritoneal dialysis session without developing peritonitis (n== 36). 2. Group II: patients who developed peritonitis during or after the peritoneal dialysis session (n= 24). 3. Group III: comprised 20 uremic patients who were on regular hemodialysis therapy (n= 20). In these groups of uremic patients, the oxidative stress (OS) was evaluated, indirectly, by measuring the changes in the following parameters: plasma MDA-LM, plasma thiol, plasma albumin, plasma uric acid and plasma bilirubin, before and after the dialysis session, and by comparing the results of the three studied groups with each other. The above parameters were also studied in a control group of (20) healthy subjects comparable in age and sex. In chronic renal failure (CRF) patients, the pre-dialysis plasma MDA-LM level was significantly higher, and the pre-dialysis plasma thiol level was significantly lower compared to the control group levels. The post-dialysis plasma MDA-LM concentration had been significantly decreased associated with a significant increase in the post-dialysis plasma thiol concentration in patients of groups I and IH as compared to their pre-dialysis concentrations. From these findings, an increased OS in patients with CRF can be most likely concluded, which was decreased after dialysis therapy. In contrast, patients who developed peritonitis (group II), had a significant increase in the post-dialysis MDA-LM concentration, associated with a significant decrease in the post-dialysis plasma thiol level, compared to the corresponding pre-dialysis and control group levels, which suggests an exacerbation of the OS in these patients/ In patients on regular hemodialysis therapy (group III), the pre-dialysis plasma MDA-LM level was significantly lower and the pre-dialysis plasma thiol level was higher, although not significantly, than the corresponding pre-dialysis values of group I. This finding suggests a minor OS in these patients compared to patients of group I. Results of changes in plasma albumin, bilirubin and uric acid were not conclusive in demonstrating the role of OS in CRF. Finally, a significant correlation was found between the OS, measured by the plasma MDA-LM concentrations and the degree of renal insufficiency, measured by serum creatinine concentrations in (40) uremic patients prior to the start of peritoneal dialysis therapy.