Heart transplant patients on a standard care regimen of CNI, MMF, and corticosteroids will enter the study 4-6 weeks post-transplant. At 3 months after transplant, patients will be randomized to either continue this regimen or CNI therapy will be discontinued and replaced by sirolimus therapy (in combination with MMF and corticosteroids). The effect of these 2 regimens on efficacy, safety and renal function will be evaluated.The anticipated time on study treatment is 1-2 years and the target sample size is 500+ individuals.