In order to assess the oxidative stress in newly diagnosed children with primary nephrotic syndrome (PNS), we serially measured serum total antioxidant capacity (TAC) and malondialdehyde (MDA) in 33 children with PNS and ten healthy matched controls. Patients were classified into two groups: those who had steroid-sensitive nephrotic syndrome (SSNS; n = 26) and those who had steroid-resistant nephrotic syndrome (SRNS; n = 7). Of the patients with SSNS, 15 were non-relapsers and 11 were relapsers. At the proteinuric phase, all patients had significantly higher MDA levels and lower TAC than the controls. These changes were more marked in patients with SRNS than in those with SSNS. During remission and still on corticosteroids, patients had higher TAC and similar MDA levels as in the proteinuric phase, but the TAC and MDA levels still significantly differed from those of the controls. More improvement in TAC and MDA levels occurred in patients following the weaning of corticosteroids, but TAC was still lower in the patients than in the controls. Moreover, TAC was higher in non-relapsers than in relapsers. Using a receiver operating characteristic curve, the initial response to corticosteroids could be predicted at serum TAC level > or =0.73 mM/L (sensitivity 89%, specificity 86%), while serum TAC levels < or = 1.14 mM/L after the weaning of corticosteroids could predict that the patient would not relapse (sensitivity 91%, specificity 80%). In conclusion, based on our results, PNS can be considered to be associated with oxidative stress even during remission. This stress may modulate the response to corticosteroids. Further prospective studies using larger numbers of patients are needed to validate these results.