Background <p>Heart transplantation (HTX) is the standard therapy for end-stage heart failure; however, reperfusion after ischemia can contribute to myocardial injury. Neutrophil infiltration and release of neutrophil elastase-related serine proteases are associated with adverse graft outcomes. Cathepsin C inhibition blocks neutrophil elastase-related protease activation. We hypothesized that the cathepsin C inhibitor BI-9740 improves graft function post-HTX.</p> Methods <p>In a rat model of heart transplantation, Lewis recipients received orally placebo (<i>n</i> = 12) or BI-9740 (<i>n</i> = 11, 20 mg/kg) daily for 12 days. Donor hearts were explanted, preserved in a cardioplegia (60 min) and heterotopically implanted (30 min), so that the total ischemia time was 90 min. Left ventricular (LV) graft function was assessed after 1 h of reperfusion. Neutrophil serine protease activity was determined in bone marrow lysates. Myocardial morphological changes were examined and immunohistochemistry and western blots were performed.</p> Results <p>Treatment with BI-9740 markedly reduced neutrophil elastase-related proteolytic activity in bone marrow lysates. Histopathological lesions, cathepsin C and myeloperoxidase-positive cell infiltration, nitrotyrosine immunoreactivity and poly(ADP-ribose)-polymerase (PARP)-1-positive cells were decreased in treated hearts. The use of BI-9740 improved graft systolic (LV systolic pressure 110 ± 6 vs. 74 ± 6 mm Hg; dP/dt_max 2782 ± 149 vs. 2076 ± 167 mm Hg/s; <i>p</i> &lt; 0.05) and diastolic function and shortened graft rebeating time compared to placebo.</p> Conclusion <p>Treatment with BI-9740 improves graft function after heart transplantation.</p>