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Objective: To compare the myocardial perfusion reserve index (MPRI) measured during stress cardiac magnetic resonance imaging (MRI) with regadenoson in patients with heart transplants versus in patients without heart transplants. Material and methods: We retrospectively compared 20 consecutive asymptomatic heart transplant patients without suspicion of microvascular disease who underwent stress cardiac MRI with regadenoson and coronary computed tomography angiography (CTA) to rule out cardiac alto graft vasculopathy versus 16 patients without transplants who underwent clinically indicated stress cardiac MRI who were negative for ischemia and had no signs of structural heart disease. We estimated MPRI semiquantitatively after calculating the up-slope of the first-pass enhancement curve and dividing the value obtained during stress by the value obtained at rest. We compared MPRI in the two groups. Patients with positive findings for ischemia on stress cardiac MRI or significant coronary stenosis on coronary CTA were referred for conventional coronary angiography. Results: More than half the patients remained asymptomatic during the stress test. Stress cardiac MRI was positive for ischemia in two heart transplant patients; these findings were confirmed at coronary CTA and at conventional coronary angiography. Patients with transplants had lower end-diastolic volume index (59.3 +/- 15.2 ml/m(2) vs. 71.4 +/- 15.9 ml/m(2) in those without transplants, p = 0.03), lower MPRI (1.35 +/- 0.19 vs. 1.6 +/- 0.28 in those without transplants, p = 0.003), and a less pronounced hemodynamic response to regadenoson (mean increase in heart rate 13.1 +/- 5.4 bpm vs. 28.5 +/- 8.9 bpm in those without transplants, p<0.001). Conclusion: Stress cardiac MRI with regadenoson is safe. In the absence of epicardial coronary artery disease, patients with heart transplants have lower MPRI than patients without transplants, suggesting microvascular disease. The hemodynamic response to regadenoson is less pronounced in patients with heart transplants than in patients without heart transplants. (C) 2020 SERAM. Published by Elsevier Espana, S.L.U. All rights reserved.