The Quilty lesion is the moniker of a lymphoid proliferation that resides predominantly in the endocardium and underlying myocardium within transplanted hearts. It has been described in the endocardium, myocardium, and epicardium. Although its pathogenesis is not understood, it is generally considered that the Quilty lesion is not a manifestation of acute rejection. However, its clinical significance and relation to acute rejection, such as its pathogenesis, are still not completely understood. Immunohistochemical studies distinguish between the cellular infiltrates of Quilty lesions and those associated with acute rejection. We describe, what we believe to be, the first case of a Quilty lesion in an epicardial coronary artery from an 8-year-old girl with congenital heart disease who required retransplantation 25 months after her cardiac transplantation for severe graft coronary artery disease. The lesion consisted of an intramural nodular aggregate of numerous T and B lymphocytes, with fewer macrophages, characteristic of a Quilty lesion.