Accelerated arteriosclerosis is the major long-term complication of cardiac transplantation. It has been demonstrated recently that accelerated arteriosclerosis is caused, in part, by rejection-related, cell-mediated immunity. However, the role of cytotoxic T lymphocytes in this process is a subject of controversy. Perforin is a specific marker of functionally active cytotoxic lymphocytes because it is a functional component of the cytotoxic granules of these cells. We examined 11 coronary arteries from seven autopsied and four retransplanted heart transplant recipients for the presence of perforin-containing lymphocytes. Immunohistochemical stains for perforin were performed using a monoclonal antibody against human perforin. Eight of the 11 coronary arteries examined were found to contain perforin-positive cells. These perforin-positive cells were present in subendothelial spaces of the coronary arteries, and the staining seen was cytoplasmic and granular. The granules often were polarized to the endothelial surface. Furthermore, the cells identified were usually in close proximity to, or in direct contact with, coronary artery endothelial cells. These results suggest that cell-mediated endothelial injury by perforin-positive cytotoxic lymphocytes may contribute to the development of accelerated arteriosclerosis in heart transplant recipients.