Diffuse myocardial fibrosis is associated with adverse outcomes, although detection and quantification is challenging. Cardiac MR relaxation times mapping represents a promising imaging biomarker for diffuse myocardial fibrosis. To investigate whether relaxation times can detect longitudinal changes in myocardial tissue composition associated with diffuse fibrosis in patients with severe aortic stenosis (AS) before and after aortic valve replacement (AVR). Prospective longitudinal study. Fifteen patients with severe AS. 3T / 3(3)3(3)5-MOLLI, T2 -GraSE, and 3D-QALAS. Patients underwent MR examinations at three timepoints: before AVR, as well as 3 and 12 months after AVR. Data from each patient was analyzed in 16 myocardial segments. The segment-wise T1 and T2 data were analyzed over time after surgery using linear mixed models for repeated measures analysis. The results showed that T1 relaxation times were significantly (P < 0.05) shorter 3 and 12 months postoperative than preoperative and that the T2 relaxation times were significantly (P < 0.05) longer 3 and 12 months postoperative than preoperative for both 3D and 2D mapping methods. No significant changes were seen between 3 and 12 months postoperative for any of the methods (P = 0.06/0.19 for T1 with 3D-QALAS/MOLLI and P = 0.09/0.25 for T2 with 3D-QALAS/GraSE). We demonstrated that changes in myocardial relaxation times and thus tissue characteristics can be observed within 3 months after AVR surgery. The significant changes in relaxation times from preoperative examinations to the follow-up may be interpreted as a reduction of interstitial fibrosis in the left ventricular wall. 1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018. © 2018 International Society for Magnetic Resonance in Medicine.