METHODS The safety of long-term immunosuppression with cyclosporine in renal-transplant recipients is not well understood. This drug may cause a progressive toxic nephropathy, but it also preserves renal function because it prevents rejection. To determine the effect of cyclosporine on renal function and graft rejection, we conducted a retrospective analysis of data on 1663 renal-transplant recipients at six centers. RESULTS The rate of graft survival was 78 percent (median follow-up, 36 months). Grafts were was lost in 279 patients (17 percent), mostly because of acute rejection (68 patients) or chronic graft dysfunction that was unresponsive to a reduction in the dose of cyclosporine (125 patients); 92 patients died with functioning grafts. The median change in the serum creatinine concentration in all patients after transplantation was less than 0.001 mg per deciliter per month (< 0.09 mumol per liter per month). Patients who had episodes of rejection had decreased rates of long-term graft function and survival. Eight percent of patients with functioning grafts at one year had first episodes of rejection more than one year after transplantation. These late first rejections were associated with noncompliance with therapy (in 34 percent), blood cyclosporine concentrations that were marginally lower than those of patients who had no episodes of rejection, and a low rate of successful reversal of rejection (77 percent, vs. 97 percent in patients with rejection during the first year; P < 0.001). CONCLUSIONS The majority of renal-transplant patients tolerate long-term cyclosporine therapy without evidence of progressive toxic nephropathy. Graft failure is most often due to rejection.