Cyclosporine (CsA) has become well established as a potent immunosuppressive agent in the renal transplantation. However, therapy is complicated by large intraindividual and interindividual variability in pharmacokinetics of CsA and frequent undesirable clinical outcomes such as graft rejection and nephrotoxicity. The objective of this study was to determine the CsA trough blood concentrations that were associated with acute graft rejection and renal toxicity in renal transplant patients. Also, the ability of the current recommendation of therapeutic range for CsA to prevent graft rejections and CsA-associated renal toxicity was assessed. The clinical courses of the patients on CsA as an immusuppressive agent for preventing the graft rejection with renal ransplantation performed at Seoul National University Hospital from January 1995 to September 1998 were retrospectively reviewed. Total of 78 patients were included and three of them were retransplantation cases. Twenty-two acute episodes of rejection were identified, but only 16 episodes were clinically significant. Of these all the episodes occurred during the first month after transplantation except one. Mean daily doses of CsA were $427.2pm72.1,;352.6pm56.8,;308.62pm48.3;and;268.47.1;mg$ at posttransplant 1, 3, 6, and 12 months, respectively. Mean CsA whole blood though levels were $259.8pm36.2,;238.5pm39,;200.8pm45.8;and;161.9pm25.8;ng/ml$ at posttransplant 1, 3, 6 and 12 months, respectively. Mean daily doses/weight were $7.9pm1,;6.4pm1,;5.3pm0.7;and;4.6pm0.7;mg/kg$ at posttransplant 1, 3, 6 and 12 months, respectively. CsA doses decreased significantly as months progressed (p<0.001). During the first month after transplantation, only $12.5\%$ of the patients in rejection group had CsA concentration in therapeutic range, and 87.5, 93.8, and $100\%$ were within the therapeutic range at posttransplant 3, 6, and 12 months, respectively. These results suggested that CsA concentrations of $250sim300;ng/ml$ might be appropriate for preventing the acute rejection during the first posttransplant month.