Nephrotoxicity is the most important side effect of cyclosporin therapy. Glomerular filtration rate is reduced in almost all patients and improves when cyclosporin is discontinued. Longterm studies in renal transplant recipients indicate that there is no progressive loss of renal function in the majority of patients treated with cyclosporin. Similar results are found in non-transplant recipients treated with low-dose cyclosporin. Approximately 10% of heart and heart-lung transplant patients develop a progressive deterioration in renal function which may not respond to stopping cyclosporin therapy.