BACKGROUND Microemulsion cyclosporine is a new pharmaceutical form whose intestinal absorption is more constant, resulting in a better bioavailability. OBJECTIVE To assess dose adjustments and variability of blood levels after the conversion of cyclosporine to microemulsion cyclosporine in renal transplant recipients function receiving cyclosporine-ketoconazole. METHODS Thirty four patients with more than one year after the transplantation, with stable renal function and receiving triple immunosuppression were studied. Conventional cyclosporine was changed to the microemulsion form maintaining the same daily dose. Drug serum levels, serum creatinine and blood pressure were measured within two to eight months after the conversion. Doses of microemulsion cyclosporine were adjusted to achieve serum levels of 150 +/- 40 ng/ml. RESULTS Microemulsion cyclosporine induced a slight initial increase in blood cyclosporine levels. Afterwards, levels were more stable than with conventional cyclosporine (165-185 and 145-210 ng/ml respectively) and the dispersion of values were lower (standard deviations of 70 and 100 ng/ml respectively). Twenty three patients did not require dose adjustments, in four it was reduced and in five it was increased. There were no changes in serum creatinine or blood pressure after the conversion. CONCLUSIONS More stable serum levels without adverse reactions were obtained with microemulsion cyclosporine. Doses of cyclosporine need not to be changed during the conversion.