In an open clinical trial we assessed the tolerance and safety of the 1:1 conversion of traditional cyclosporine A (CyA) to a new cyclosporine formulation based on a microemulsion technology (CyN) in 18 patients with stable renal allografts. 56% patients were female. Median patient age was 40.9 +/- 3.2 years (range 18 to 65). Renal transplantation was performed in 24.1 +/- 4.6 months (range 6 to 67 months), prior to the beginning of the study, and 67% of the transplants were from cadaveric donor. The most frequent underlying renal disease was glomerulonephritis (44.4%). None of the patients entering the study were withdrawn prematurely. After 2 weeks of observation for graft function stability, the study was divided in two phases: I: during 4 weeks the patients received CyA traditional at fixed doses (Mean dose administered 3.056 +/- 0.25 mg/Kg/d) and II: during the consecutive 6 weeks with conversion to CyN, with doses adjustment as required (Mean dose 2.887 +/- 0.21 mg/Kg/d). Clinical events, adverse reactions and laboratory parameters were evaluated. Levels of 100-200 ng/ml measured by monoclonal specific fluorescence polarization immunoassay were considered appropriate. There were no significant changes in physical examination and laboratory parameters between phases. The incidence of adverse reactions reported in phase I was only gingival hypertrophy (5%) which persisted in phase II, qualified as probably related to the cyclosporine, and in phase II tremor in 17%, qualified as definitively related. Both drugs were well tolerated and there was no report of acute rejection during the study. We conclude that the tolerance and safety of the 1:1 conversion of CyA to CyN were confirmed by our results, and considering the improved pharmacokinetic properties of the second, the microemulsion presentation will be used preferentially as immunosuppressive drug in the treatment of stable kidney transplant patients.