Seven patients with minimal-change nephrotic syndrome confirmed by renal biopsy were treated with cyclosporin (CsA). Four patients had frequent relapses and three others had primary steroid resistant nephrotic syndrome. Corticosteroids were discontinued as soon as CsA whole blood trough values of 200-500 ng/ml (RIA method) were reached. A full remission, defined as complete disappearance of proteinuria, was achieved in five patients under this treatment. In the two other patients proteinuria was reduced. Two patients experienced an acute episode of dose-dependent nephrotoxicity; however, overall renal function, as determined by the creatinine clearance, was stable. Control biopsies in five patients after a mean treatment period of 10 months showed no significant vascular or interstitial toxicity.