Background: Neopterin (NP) is a pyrazino-pyrimidine compound derived from guanosie 5'-triphosphate. NP production has been linked to the activation of cell-mediated immunity. Increases in serum NP concentration :md urinary NP excretion have been used as diagnostic or prognostic markers in various immune-mediated clImcal conditions including allograft rejection, infectious diseases, autoimmune diseases, and neoplastic dIseases. Objecthes: To assess the changes that occur in serum neopterin levels in children with primary nephrotic s: ndrome (PNS) 'Iethods: Scmm ~'P levels were measured by ELISA in 38 children with active PNS (27 males, II females: aged 6.~ ± 2.9 years) (Group I) and 17 children (II males, 6 females; aged 7 ± 2.5 years) with PNS in rCllllssion (Group [I) and 20 age-and sex-matched controls. All patients had normal creatinine clearance. Among Group I children, 28 patients were steroid-sensitive (SSNS) while 10 patients were steroid-resistant ( SR:S) Results: Semm NP levels were significantly elevated in Group I patients (median 7.5, range .8-10.8 ng/ml) compared to Group II (median = I range 0.5-2.5 nglml, p < 0.00 I) and controls (median = 0.6, range 0.1-2.7 ng!mL p 0.00 I). Group II patients had similar NP levels compared to controls (p = 0.71). There was a significant positive correlation between serum J\'P levels and the degree of proteinuria in Group I patients (r OA, p = 0(1). No significant differences in serum NP levels were noted between SSNS and SRNS patients (p 0.4). Conclusions: St)rum 1'.TP could be used as a marker of the activity of PNS but it could not be used to differentiate between SSNS and SRNS.