The pathology of heart allograft rejection. The pathologist plays an important role in the care of cardiac transplant recipients. Day-to-day management of immunosuppression is largely dependent on the diagnosis and grading of acute rejection. While noninvasive methods have been tried experimentally, the endomyocardial biopsy remains the gold standard in monitoring the rejection status of the heart allograft. The diagnosis of rejection, however, is complicated by a variety of other histologic findings, which may be procedural or processing related, due to sampling, or specifically related to transplantation. Failed allografts, whether from autopsy or explantation, provide the opportunity to study short- and long-term changes in the transplanted heart. Allograft arteriopathy, or graft atherosclerosis, is the major limiting factor in long-term recipient survival. While the morphological features of graft arteriopathy have been well described, the mechanism and factors contributing to its development remain unclear. 