ObjectivesThis study analyzed the prevalence and pattern of focal andpotentialdiffuse myocardial fibrosis detected by late gadolinium enhancement (LGE) and extracellular volume (ECV) imaging in male and female marathon runners using cardiac magnetic resonance (CMR).MethodsSeventy-four marathon runners were studied including 55 males (44 ± 8 years) and 19 females (36 ± 7 years) and compared to 36 controls with similar age and sex using contrast-enhanced CMR, exercise testing, and blood samples.ResultsContrast-enhanced CMR revealed focal myocardial fibrosis in 8 of 74 runners (11%). The majority of runners were male (7 of 8, 88%). LGE was typically non-ischemic in 7 of 8 runners (88%) and ischemic in one runner. ECV was higher in remote myocardium without LGE in male runners (25.5 ± 2.3%) compared to male controls (24.0 ± 3.0%,p< 0.05), indicating the potential presence of diffuse myocardial fibrosis. LV mass was higher in LGE + males (86 ± 18 g/m2) compared to LGE- males (73 ± 14 g/m2,p< 0.05). Furthermore, LGE + males had lower weight (69 ± 9 vs 77 ± 9 kg,p< 0.05) and shorter best marathon finishing times (3.2 ± 0.3 h) compared to LGE- males (3.6 ± 0.4 h,p< 0.05) suggesting higher training load in these runners to accomplish the marathon in a short time.ConclusionThe high frequency of non-ischemic myocardial fibrosis in LGE + male runners can be related to increased LV mass in these runners. Furthermore, a higher training load could explain the higher LV mass and could be one additional cofactor in the genesis of myocardial fibrosis in marathon runners.Key Points• A high frequency of myocardial fibrosis was found in marathon runners.• Myocardial fibrosis occurred typically in male runners and was typically non-ischemic.• Higher training load could be one cofactor in the genesis of myocardial fibrosis in marathon runners.