Little data have been published on tubular renal function during cyclosporin A treatment in children without transplants. We studied 12 young subjects (mean age 10 years) with steroid-responsive idiopathic nephrotic syndrome and with signs of steroid toxicity. After achieving remission with prednisone 60 mg/m2, 8 children started cyclosporin A therapy (6 mg/kg/day) (group A) and 4 children were given cyclophosphamide 2.5 mg/kg/day (group B). The latter were considered as controls together with 10 other children with idiopathic nephrotic syndrome in complete remission and off therapy (group C). We monitored creatinine clearance and tubular handling of beta 2-microglobulin, sodium, phosphorus and uric acid for one year. Cyclosporin A induced a decrease in creatinine clearance with a decrease in fractional excretion of beta 2-microglobulin; sodium excretion was similar in the two treated groups and a transient decrease in fractional excretion of uric acid was seen only in patients receiving cyclosporin A. Both groups showed an increased renal threshold phosphate concentration. Our results suggest that in children, cyclosporin A therapy induces a decrease in glomerular filtration rate associated with increased reabsorption activity of proximal tubular cells.