In a pilot study 23 children with nephrotic syndrome were treated with cyclosporin A (Cs) for 6-45 months. 8 children suffered from steroid dependent minimal change nephrotic syndrome (MCNS) and had experienced at least one course with cytotoxic drugs, but had relapsed thereafter. 2 children had diabetes mellitus type I with nephrotic syndrome and 13 children had steroid resistant focal segmental glomerulosclerosis (FSGS). Cs was started with 100 mg/m2/day in two doses and increased stepwise to obtain a Cs whole blood trough level of 200-400 ng/ml. In steroid dependent MCNS treatment with Cs reduced relapse rate significantly, and prednisone therapy could be stopped completely. After discontinuation of Cs, relapses reoccurred as frequently as before. Renal function remained unimpaired despite repeated Cs treatment courses up to 38 months. In cases of nephrotic syndrome with diabetes type I Cs treatment led to complete remission without changing the insulin requirement. However, after discontinuation of Cs relapses reoccurred. In steroid resistant FSGS 6 children benefited from Cs treatment: 4 went into complete remission, 2 into partial remission. The 2 children with complete remission relapsed but remained Cs responsive. The remaining 7 children with FSGS did not respond to Cs but continued the course of their disease, with two patients rapidly progressing to terminal renal failure. Side-effects of Cs treatment were mild. It is concluded that Cs is an effective agent in steroid dependent MCNS and can be used as an alternative drug in specific cases like steroid toxicity or diabetes mellitus. In steroid resistant FSGS a trial with Cs seems to be warranted since some cases do respond favorably. To avoid nephrotoxicity treatment with Cs should always be monitored closely by determination of blood levels and renal function.