Cyclosporin A (CyA, Sandimmun) was given to 71 children with idiopathic nephrotic syndrome, 45 of whom were steroid-dependent with signs of steroid toxicity, and 23 who were steroid-resistant. Cyclosporin A was effective in 80% of the steroid-dependent patients, allowing cessation of corticosteroid treatment. However, most of these patients relapsed when CyA was tapered or withdrawn. In these patients, CyA may be required for long periods of time. Conversely, CyA was less effective in steroid-resistant patients; only 7% achieved remission with CyA alone. Cyclosporin A in association with prednisone may be an effective alternative as 8 out of 14 patients entered remission with this treatment combination. Serial renal biopsies were performed in 43 patients to evaluate the potential nephrotoxicity of the treatment. Eighteen patients developed significant tubulointerstitial lesions which were attributable to CyA nephrotoxicity. The risk of developing chronic nephrotoxicity appears to be higher in steroid-resistant patients, and was not related to the duration of treatment. Cyclosporin A nephrotoxicity can develop in patients with normal renal function.