Cardiac allograft rejection remains a problem, despite advances with immunosuppressants. Understanding the mechanisms behind rejection is essential for developing targeted therapies. The goal of this investigation is to explore Sirtuin 1 (Sirt1) as a therapeutic target for cardiac allograft rejection. Thirteen endomyocardial biopsy specimens with acute cellular rejection (grade 2R or 3R) were selected. CD3, CD4, CD8, CD20, CD68, T-cell intracytoplasmic antigen (TIA-1), and Sirt1 expressions were determined by immunohistochemical stains. Comparison of Sirt1 expression was made with 10 cases of grade 0R and grade 1R. Quantitative image analysis was performed. There were 2 cases of grade 3R and 11 cases of grade 2R acute cellular rejection. Sirtuin 1 expression was present in the majority of mononuclear cells (median percentage, 73.5; interquartile range, 51.2-100%); staining was also observed in cardiomyocytes. Twelve of the 13 cases (92.3%) had an elevated CD8/FoxP3 ratio, coinciding with acute cellular rejection. Sirtuin 1 expression in the nuclei of FoxP3+ cells can lead to deacetylation and inactivation of FoxP3 rendering the T-suppressor cells inactive and promoting acute cellular rejection. The use of a Sirt1 inhibitor may be a therapeutic option in expanding the functionality of the FoxP3+ T-suppressor cells and moderating the severity of such rejection.