['The purpose of this study was to investigate diffuse myocardial fibrosis in patients with systolic heart failure (SHF) and in patients with heart failure with preserved ejection fraction (HFpEF) and the association with diastolic dysfunction of the left ventricle (LV).', 'Increased diffuse myocardial fibrosis may impair LV diastolic function. However, no study has verified the association between the degree of diffuse myocardial fibrosis and the severity of impaired diastolic function in SHF and HFpEF.', 'Forty patients with SHF, 62 patients with HFpEF, and 22 patients without HF underwent cardiac magnetic resonance (CMR), including T1 mapping and cine CMR on a 3-T system. Extracellular volume fraction (ECV), a measure of diffuse myocardial fibrosis, was quantified from T1 mapping. Systolic and diastolic functions of the LV were assessed by cine CMR. The ECV values and LV functional indexes were compared among the 3 groups. Associations between ECV and LV diastolic function were also investigated.', 'Compared with patients without HF, significantly higher ECV was found in patients with SHF (31.2% [interquartile range (IQR): 29.0% to 34.1%] vs. 27.9% [IQR: 26.2% to 29.4%], p < 0.001) and HFpEF (28.9% [IQR: 27.8% to 31.3%] vs. 27.9% [IQR: 26.2% to 29.4%], p = 0.006). Peak filling rate, a diastolic functional index assessed by cine CMR, was significantly decreased in patients with SHF (1.00 s(-1) [IQR: 0.79 to 1.49 s(-1)] vs. 3.86 s(-1) [IQR: 3.34 to 4.48 s(-1)], p < 0.001) and HFpEF (2.89 s(-1) [IQR: 2.13 to 3.50 s(-1)] vs. 3.86 s(-1) [IQR: 3.34 to 4.48 s(-1)], p < 0.001). Myocardial ECV was significantly correlated with peak filling rate in the HFpEF group (r = -0.385, p = 0.002), but no correlation was found in the SHF and non-HF groups (r = 0.030, p = 0.856 and r = -0.238, p = 0.285, respectively).'] Is cMR-verified diffuse myocardial fibrosis associated with diastolic dysfunction in HFpEF?