Objective To investigate the Th1/Th2 cells imbalance in childhood primary nephrotic syndrome(PNS)and its relation to disease activity. Methods Percentage of T-helper-1(Th1) and T-helper-2 (Th2) cells was examined by three-color flow-cytometry in 21 cases of first-onset PNS, 16 cases of remission PNS and 10 healthy children. Results The percentage of Th1, Th2 and Th0 cells in healthy children was (13. 42?4. 36)% , (2. 53 ? 1. 97)% and (1. 25?0. 92)% respectively. In the patients with first-onset PNS, the percentage of Th1, Th2 and Th0 cells was significantly decreased as compared to healthy children [(2. 34 ?2. 09)%, (1.02?0.96)% and (0.40?0.38)% respectively, all P 0. 05]. Because of relatively low percentage of Th1 cells in contrast to Th2 cells in the patients with first-onset PNS, Th1/Th2 ratio was also significantly reduced in those patients, as compared to the patients with remission PNS and the healthy children (2. 43 ? 2.65 vs 4. 17?2. 32 and 5. 41?2. 77, both P