Severe diastolic dysfunction has important clinical implications in advanced systolic heart failure. The authors investigated whether a marker of fibrosis, serum carboxy‐terminal peptide of procollagen type I (PICP) is a major determinant of diastolic function in 40 patients with heart failure and ejection fraction <35%. Patients with unstable heart failure or ischemic symptoms were excluded. The authors found PICP to be an independent predictor of diastolic function in addition to age and pulmonary artery systolic pressure. The authors’ findings suggest that studies evaluating whether therapy that improves myocardial fibrosis could have a favorable impact on diastolic function in this population are warranted.