Cyclosporin A (CsA) is an effective immunosuppressive drug for the prophylaxis of rejection after organ transplantation. However, CsA is potentially toxic to various tissues: kidney, liver, pancreas, nervous system, etc. The aim of this study was to ascertain the frequency of CsA nephrotoxicity incidence according to the changes in graft biopsy material and its association with whole blood CsA levels. Data were obtained from 30 recipients after cadaver or living related kidney transplantation. All patients (pts) were divided into two groups: Gr1 included 17 pts with biopsy evidence of CsA damage and Gr2 -13 pts without these changes. The mean age of recipients (38.6+/-11.3 vs 34.6+/-13.3), donor and recipient human leucocyte antigen (HLA) match (2.5/6 vs 2.3/6), cold ischemic time (12.0+/- 9.3 h vs 13.8+/-10.3 h), percentage of kidney from cadaver donors (64.7% vs 69.2%) were similar in both groups. Comparison of CsA blood levels (>200 ng/ml) between Gr1 and Gr2 revealed statistically significant differences (70.6% vs 15.4%, p<0.05), correspondingly. The mean CsA blood level was higher in Gr1 (328.7+/-153.8 ng/ml vs 202.4+/-145.6 ng/ml, p<0.03). Thus, we suggest that CsA nephrotoxicity is associated with elevated CsA levels of more than 200 ng/ml. Biopsy is a very important criteria that helps to distinguish CsA nephrotoxicity and acute rejection.