Nephrotoxicity and cost are the major problems in the use of cyclosporine (CsA) in renal transplantation. Thus, maintenance of CsA levels at the lower limits of the therapeutic range is desirable. The lowest CsA level effective in preventing rejection while avoiding nephrotoxicity has not been defined. We report on 44 primary cadaveric renal transplant recipients treated with a protocol that involved a progressive reduction in the trough CsA levels. CsA was initiated at an oral dose of 15 mg/kg, and this dose was adjusted to achieve serum trough levels, as measured by radioimmunoassay, of 150-200 ng/ml during the first month, 100-150 ng/ml during the second month, 75-100 ng/ml during the third month, and 50-75 ng/ml thereafter. Patient and graft survival at 18 months were 94% and 83.6%, respectively. The mean daily CsA doses were 6.7 +/- 3.1 mg/kg at 6 months, 5.5 +/- 3.2 mg/kg at 12 months, and 4.7 +/- 2.4 mg/kg at 18 months. Corresponding trough serum CsA levels were 94 +/- 59 ng/ml, 64 +/- 22 ng/ml, and 44 +/- 21 ng/ml at 6, 12, and 18 months, respectively. Mean serum creatinine concentrations were 1.8 +/- 0.6 mg/dl at 6 months, 1.7 +/- 0.5 mg/dl at 12 months, and 1.6 +/- 0.5 mg/dl at 18 months. The mean serum creatinine concentration at 18 months was not significantly different from that of 18 conventionally treated primary cadaveric renal transplant recipients (1.6 +/- 0.5 vs. 1.4 +/- 0.4 mg/dl, P = .31). A total of 67% of patients did not have any rejection episodes under this protocol, while 71% of patients never developed CsA nephrotoxicity. No patient was taken off CsA for progressive nephrotoxicity. We conclude that trough serum CsA levels of 50-75 ng/ml, as measured by radioimmunoassay, are sufficient to maintain effective immunosuppression in the long-term management of primary cadaveric renal transplant recipients. These values are much lower than previously recommended, and this approach ameliorates chronic CsA nephrotoxicity.