The aim of this study was to determine the association of acute allograft dysfunction during the first 6 posoperative months with serum levels of cyclosporine 12 hours (C0) and 2 hours (C2) after administration. The C0 and C2 levels were recorded in 65 kidney recipients at 1 week and 1, 2, 3, and 6 months postoperatively. These levels were evaluated in association with creatinine rise, acute rejection, and complications. The mean dose of cyclosporine was 451 mg, 350 mg, 275 mg, 235 mg, and 210 mg, at 1 week and 1, 2, 3, and 6 months, respectively. Allograft dysfunction was seen in 30 patients (46.2%) due to acute rejection in 7.7%, cytomegalovirus infection in 4.6%, urologic disorders in 6.2%, and cyclosporine toxicity in 30.7%. The C0 level was associated with cyclosporine nephrotoxicity (P=.006), but not with acute rejection. The C2 level had no predictive value. Fifty-five (84.6%) patients experienced complications related to cyclosporine and C0 levels was associated with these complications (P=.026). Monitoring of patients who receive cyclosporine by C0 can help us control drug-related nephrotoxicity. The high frequency of cyclosporine side effects while administration of standard doses of drug indicates that we should consider a modification in the basic dose of cyclosporine in our patients.