Cyclosporin A (CsA) in combination with 6-methylprednisolone was administered over a six-month period to 10 children with nephrotic syndrome (6 frequently relapsing and 4 steroid resistant) at a dose providing blood CsA concentrations of 200-400 ng/ml. In frequent relapsers 6-methylprednisolone was given initially at a dose of 60 mg/m2/day and subsequently at a dose of 35 mg/m2/48 h. In steroid resistant cases the steroid was given at an initial dose of 1 mg/kg tapering to 0.35 mg/kg by the end of the first month and to 0.22 mg/kg by the fourth month being also administered at the same dosage during the fifth and the sixth month. Among the frequent relapsers there were 30 relapsers in 116 months before CsA and 1 relapse during 70 months after CsA (in 5 patients urine was protein free and 1 relapsed after 7 months. Two of the steroid resistant cases healed. There was a modest incidence of CsA toxicity as evidenced by the usual clinical monitoring and by evaluation of creatinine clearance, plasma potassium and evolution of renal parenchymal cells as studied in the course of sequential fine needle aspiration biopsies.