In search of a non-invasive alternative detection of early-stage cardiac allograft vasculopathy (CAV), in this preliminary study we tested the hypothesis that interstitial fibrosis quantified with cardiac magnetic resonance (CMR) can serve as a biomarker for the detection of CAV. Late-stage CAV was detected with routine X-ray coronary angiography (XRCA), while a coronary intima-media thickness ratio (IMTR) > 1 on optical coherence tomography (OCT) was used to detect early-stage CAV. Interstitial fibrosis was quantified in the endomyocardial biopsy (EMB) and indirectly with CMR as the T₁ relaxation time and extracellular volume (ECV). CMR was performed within 48 h of a single invasive procedure with XRCA, OCT, and EMB procurement in stable HTx recipients (n = 27; age 54 ± 13 years, 5.4 ± 3.7 years post-transplant). XRCA-CAV and IMTR > 1 were present in 15% and 75% of study patients, respectively. The T₁ relaxation times and ECV were increased in patients with XRCA-CAV (p = 0.03 each), while IMTR and EMB interstitial fibrosis were not significantly different (both p > 0.05). ECV (ρ = 0.46, p = 0.02) and IMTR (ρ = 0.58; p = 0.01) correlated with the histological quantity of interstitial fibrosis, while the T₁ relaxation time (p = 0.06) did not. The correlation of the IMTR with the EMB interstitial fibrosis tentatively validates the hypothesis that interstitial fibrosis may serve as an early indicator of CAV. Moreover, the significant association of CMR-based ECV with the magnitude of interstitial fibrosis in the biopsy suggests ECV as a potential biomarker for interstitial fibrosis due to early-stage CAV. The measurement of ECV may therefore have a role for non-invasive detection and follow-up of early-stage CAV.