OBJECTIVE Cyclosporin A (CyA) has been used in steroid-dependent and steroid-resistant nephrotic syndrome (NS) with the aim to prolong or to induce remission, respectively. METHODS The efficacy and side-effects of CyA therapy were evaluated in 25 children with idiopathic NS. Twelve patients had steroid-dependent NS and 13 patients had steroid-resistant NS. In all cases, CyA was given as a third alternative drug, once therapies with prednisone and alkylating agents had failed. In steroid-resistant patients CyA administration was always associated with low-dose prednisone. RESULTS All 12 patients with steroid-dependent NS entered into remission during CyA administration, but 7 patients relapsed when the drug was withdrawn or tapered and 7 of 8 patients requiring long-term therapy continued to present new relapses. Prednisone requirement was lower and growth velocity higher during the year on CyA therapy than during the year preceding CyA therapy. Only 5 of the 13 patients with steroid-resistant NS had a complete remission. Three of these patients relapsed upon cessation of therapy, but these relapses became steroid-sensitive. Clinical side-effects (hirsutism, gum hyperplasia, arterial hypertension) were only observed in a few patients. Biochemical side-effects (hyperuricemia, hypomagnesemia) were more frequently observed, but always reverted upon cessation of therapy. The development of osteosarcoma in one patient may represent a coincidental finding. CONCLUSIONS The results suggest that CyA therapy is capable of inducing remission in all patients with steroid-dependent NS and in about one third of patients with steroid-resistant NS. However, most patients relapse when the CyA is stopped and require long-term therapy, often associated with administration of predisone.