The long-term goal of this research is to apply novel technology for detection of donor organ (allograft) rejection to improve patient outcomes following heart transplantation. The specific goal of this study is to determine whether daily monitoring of the transplant recipient's electrocardiogram (ECG) using a simple home device with transmission to an ECG Core Laboratory would provide an early biomarker for acute rejection. Despite routine immunosuppressant drug therapy, acute rejection is common, especially within the first 6 months following transplant surgery. To detect rejection, frequent endomyocardial biopsies of heart tissue are performed. An endomyocardial biopsy is a costly and invasive procedure performed in a hospital cardiac catheterization laboratory that has associated risks. Recent evidence suggests that acute allograft rejection causes delays in ventricular repolarization resulting in a longer QT interval on the ECG. The specific aims of the study are to: 1) determine whether an increase in the QT interval during the first 6 months following heart transplant is a sensitive and specific biomarker for acute rejection; and 2) determine the timing of QT interval increases relative to biopsy-diagnosed stages of mild/moderate/severe rejection. The potential benefit of finding a simple ECG biomarker of allograph rejection that could be measured at home is that it might yield earlier detection of rejection, allow more timely therapy and reduce mortality from acute allograft rejection.