The efficacy and side effects of the oral administration of cyclosporin A (Cyc-A) were investigated in 13 children with the nephrotic syndrome. Two of these children were steroid-dependent, 9 were resistant to steroids, and 2 displayed frequent relapses. The diagnosis of 6 patients was mesangioproliferative glomerulonephritis (MezPGN), of another 6 mesangioproliferative glomerulonephritis, and of 1 minimal change disease. Cyc-A was administered for 6 months at a dosage of 7 mg/kg/day with continual monitoring of the serum levels. At the end of the treatment period, 7 of the patients (53.8 %) had a total remission, 5 (38.4%) a partial remission, and 1 (7.8%) did not respond to treatment. While total remission was achieved in 4 patients who had responded well to steroids, this was true for only 3 (33.3%) of the 9 patients who had not benefited from steroids. The effects and side effects of Cyc-A on histopathological morphology were also investigated by means of posttreatment biopsies. While histopathological improvement was seen in 3 patients with MezPGN, there was evidence of deterioration in 1 patient with MLH.