Proteinuria in children with idiopathic nephrotic syndrome is secondary to a loss of charge selectivity of the glomerular basement membrane. Loss of anionic charges may be secondary to a defect of heparan sulfate proteoglycans, which is also found in the congenital nephrotic syndrome, or to cationic proteins, which neutralize the anionic charges of the membrane. Reports on recurrence of nephrotic syndrome in patients with steroid-resistant idiopathic nephrotic syndrome following renal transplantation suggest that a humoral factor, possibly produced by T lymphocytes, may enhance glomerular permeability. Corticosteroids remain the basic treatment of idiopathic nephrotic syndrome. Most patients respond to steroid therapy and a high proportion of them relapse but continue to respond throughout the subsequent course of the disease. Levamisole may be effective in preventing relapses. Cyclosporine may be useful in steroid-dependent patients with signs of steroid toxicity and after a failure of a course of alkylating agent. Almost 85% of patients respond to cyclosporine, but they relapse after tapering or stopping the drug. In steroid-resistant patients, there is no study showing a clear-cut beneficial effect of alkylating agents, as the remission rate after treatment is close to the rate of spontaneous remission. Cyclosporine in association with prednisone may be effective, but the risk of nephrotoxicity seems to be higher than in steroid-dependent patients.