Sirolimus is a new immunosuppressive agent. This study aimed to evaluate the efficiency of sirolimus in patients after renal transplantation and to compare graft function, the frequency of rejection episodes and complications with patients under cyclosporin A treatment. From May 2002 to January 2005 26 renal transplant patients were treated with sirolimus. 13 patients (group A) were treated with sirolimus before renal transplantation and 13 patients (group B) were converted to sirolimus in late period after transplantation because of chronic cyclosporin A nephrotoxicity, chronic graft nephropathy and due to intolerance of cyclosporin A (mean time after transplantation: 18 months). Sirolimus was started as a loading dose 5-6 mg per day and reduced to 2-3 mg per day. Mean sirolimus blood concentration was 8.19+/-6.7 ng/ml. Results were compared according to age, gender, the number of HLA matches, plasma renin activity levels, etc., with 52 patients (control (C) group) under cyclosporin A, mycophenolate mofetil and steroids treatment. During 3 months, the acute rejections were in 30.8% of patients (4/13) and 65.4% of patients (17/26) for group A and group B, respectively (chi2=6.568, p<0.05). Renal function at 12 months: mean serum creatinine was 165.5+/-29 micromol/l vs. 214.2+/-67.9 micromol/l, urea 9.6+/-2.6 mmol/l vs. 13.9+/-9.3 mmol/l. There were no differences in platelet counts between groups, but serum cholesterol value was higher in the patients of group A (8.11+/-0.9 mmol/l vs. 6.54+/-1.4 mmol/l), blood pressure (140+/-13/87+/-14 mmHg vs. 150+/-15/85+/-12 mmHg). Patients were treated for different infections, cytomegalovirus infection and sepsis (28.6% (6/21) vs. 45.2% (19/52) for group A and group B, respectively). Our results have shown that sirolimus in combination with mycophenolate mofetil and steroids is an effective alternative to continuous therapy without cyclosporine.