Two Six-month pilot studies were conducted in renal (n=17) or liver (n=15) transplant recipients to evaluate renal function after conversion from calcineurin inhibitor (CI)- to sirolimus (SRL)-based immunosuppression. After an SRL loading dose, doses were individualized to achieve whole blood trough levels of 10–22 ng/mL. Overall, serum creatinine did not change from baseline to six months post-conversion but an improvement from 219.9 to 201.4 μmol/L at three months was noted in renal transplant recipients (p<0.05). Another finding was a numerical increase in the mean glomerular filtration rate (GFR) from 26.8 to 33.2 mL/min/1.73 m2 at six months among liver transplant recipients (NS). All patients survived and all grafts were functioning at the end of the study. In conclusion, renal function remained stable, with a tendency towards improvement, after abrupt conversion from CI- to SRL-based therapy in renal or liver transplant recipients with moderate renal insufficiency.