Modified Release (MR) tacrolimus is an extended release formulation of tacrolimus (Prograf) administered once daily in the morning. In healthy volunteers, the MR tacrolimus formulation given qd AM and Prograf administered twice daily (bid) have a similar exposure (AUC) and trough levels (Cmin), with a reduced peak level (Cmax). Subsequently, pharmacokinetic studies were performed in stable kidney and liver transplant recipients converted from Prograf bid to MR tacrolimus qd AM. The steady-state tacrolimus exposure and target trough level range of MR tacrolimus were equivalent to Prograf after a mg-for-mg daily dose conversion in these two groups of patients, and there is a high correlation of exposure to trough levels for both Prograf and MR tacrolimus, as well as significantly less intra-subject variability in exposure after conversion to MR tacrolimus. These results indicate that stable kidney and liver transplant recipients can be safely converted from standard Prograf twice daily dosing to the same mg-for-mg daily dose of MR tacrolimus once daily in the morning. Hopefully a once daily dosing regimen of tacrolimus can improve patient compliance while maintaining effective immunosuppression.