<h3>Purpose</h3> Elevated arterial stiffness causes abnormal transmission of pulse pressure leading to target organ damage and increased risk for cardiovascular diseases. Increased arterial stiffness and endothelial dysfunction have been hypothesized as one of the mechanisms for the development of microvascular dysfunction and coronary allograft vasculopathy (CAV) after heart transplantation (HT). We sought to assess aortic stiffness and distensibility in pediatric post-HT patients and healthy controls. <h3>Methods</h3> We performed a prospective study analyzing the ascending (donor tissue) and descending aorta (recipient tissue) using transthoracic echocardiographic M-mode measurements in healthy controls and HT patients aged <21 years. Patients with existing vascular, rheumatologic, connective tissue diseases and hypertension were excluded. The descending aorta M-mode was obtained from the subcostal long axis view and the ascending aorta M-mode was obtained from the parasternal long axis view 3-5mm above the sinotubular junction. Two independent reviewers performed the measurements over 2-3 cardiac cycles, which were averaged. We calculated Aortic Distensibility (AD) and Aortic Stiffness Index (ASI) utilizing previously validated methods as shown in Table 1. <h3>Results</h3> We recruited 27 HT patients, of which 10 had congenital heart disease and 17 had cardiomyopathy and 28 healthy controls. Study results are summarized in Table 1. There was no significant difference in demographics between groups. Diastolic blood pressure was significantly higher in the transplant group. HT patients demonstrated significantly higher ascending aortic end diastolic dimensions, ascending aortic stiffness index and lower ascending aortic distensibility compared to healthy controls. Time from transplant did not affect these parameters. In 3 patients with CAV, the stiffness parameters were comparable to post-HT patients without CAV. <h3>Conclusion</h3> Our study demonstrated higher ascending aortic stiffness and lower distensibility in post-HT patients.