Neoral (NEO) is claimed to have better pharmacokinetics than standard preparation of cyclosporine (SIM) thus providing more reliable immunosuppression. We estimated safety and tolerability of NEO and compared pharmacokinetic parameters in 20 stable renal allograft recipients (RARs) converted from SIM to NEO treatment. Another 20 stable RARs continuously treated with SIM created a control group. Whole blood through CsA level (C0) did not differ after conversion (SIM: 136.2 +/- 33 ng/ml and NEO: 142.6 +/- 34 ng/ml). During therapy with NEO peak blood concentration (Cmax) was significantly higher (935.6 +/- 368 ng/ml) and occurred earlier (Tmax 1 hr. 36 min. +/- 30 min) as compared to the period on SIM (Cmax 598 +/- 309 ng/ml, p = 0.01), Tmax = 3 hr. +/- 1 h 36 min., (p = 0.01) respectively. AUC increased from 2975.4 +/- 1020 ngxhr/ml to 4236.1 +/- 1188 ngxhr/ml (p < 0.0001). Correlation coefficient between AUC and C0 was higher during NEO (r = 0.52) than SIM therapy (r = 0.32). The only noticeable change in laboratory tests after switch to NEO was slight increase of serum triglyceride concentration (119.5 +/- 44.7 mg/dL vs. 148.4 +/- 67.0 mg/dl). The mean serum creatinine concentration did not change significantly (1.42 +/- 0.32 mg/dL and 1.46 +/- 0.31 mg/dL). Tolerance of NEO was good and 1:1 switch from SIM to NEO is clinically safe. Higher bioavailability of NEO was not associated with decreased tolerability or increased nephrotoxicity. Better correlation between C0 and AUC during NEO administration makes CsA treatment monitoring more reliable.