To investigate the distribution of myocardial fibrosis and patterns of tissue characteristics on cardiovascular magnetic resonance (CMR) between athletes with LV dilatation and mild DCM patients. We prospectively recruited male cyclists/triathletes aged ≥50y who undertook ≥10h/week of exercise for ≥15y along with age/sex-matched patients with non-ischaemic heart failure (HF). Participants underwent clinical assessment, 12-lead ECG, stress-perfusion CMR with fibrosis assessment and parametric tissue mapping.Following CMR, included participants in both groups had LVEF>40% and LVEDVi>110ml/m2 without ischaemic heart disease or significant cardiac pathology on CMR likely to cause HF.Of 113 participants (64 athletes, 49 mild DCM patients), athletes with fibrosis demonstrated a greater prevalence of inferolateral fibrosis (87.5% vs 50.0%, P=0.002) whereas inferoseptal fibrosis was more common in mild DCM patients (45.8% vs 9.4%, P=0.002).Native T1 (1249.0±38.1 vs 1308.3±47.1ms,P<0.001) and extracellular volume (ECV) (22.0±2.1 vs 25.9±3.5%, P<0.001) were lower in athletes. Athletes had greater right ventricular end-diastolic volume indexed to body surface area (RVEDVi) (121.0±14.3 vs 97.6±25.2%, P<0.001), myocardial perfusion reserve (MPR) (3.65±1.30 vs 2.76±0.92,P<0.001) and stress myocardial blood flow (MBF) (2.09±0.70 vs 1.62±0.66,P<0.001) than mild DCM patients.On receiver-operator curve analysis, native T1 (area under curve (AUC) 0.89,P<0.001), ECV (AUC 0.85,P<0.001), RVEDVi (AUC 0.81, P<0.001) and stress MBF (AUC 0.68,P=0.002) were able to differentiate between groups. Septal fibrosis is rare amongst veteran athletes with LV dilation in contrast to mild DCM patients. Native T1, ECV and RVEDVi can also discriminate between these overlapping phenotypes which may be clinically useful.