The use of cyclosporin A (Cy A) in idiopathic nephrotic syndrome, particularly lesions of focal segmental glomerular sclerosis, is controversial. A retrospective study of 10 adult patients with nephrotic syndrome treated with Cy A was performed. Histological diagnosis was established in all patients: focal segmental glomerular sclerosis (n = 6), focal global sclerosis (n = 1), mesangial proliferative glomerulonephritis (n = 1), focal proliferative glomerulonephritis (n = 1) and minimal change disease (n = 1). All patients had previously received immunosuppressive therapy (duration of steroids 1-76 months; 35.0 +/- 12.1, mean +/- SEM). Cy A in a dose of 3-5 mg/kg/day, reduced proteinuria from 16.85 +/- 6.67 to 3.37 +/- 1.48 g/24 hours (P = 0.008), with an associated increase in serum albumin from 15.2 +/- 2.6 to 34.3 +/- 2.5 g/l (P < 0.001). In six patients steroid therapy was discontinued. Cy A was well tolerated for up to 5 years. There was no significant nephrotoxicity. In conclusion, Cy A was effective treatment of refractory idiopathic nephrotic syndrome, including those cases with focal segmental glomerular sclerosis.