Aim To address the mechanisms responsible for the increase in LV filling pressures induced by acute hypertension transients in patients with heart failure with preserved ejection fraction ( HFpEF) Methods and results Multiple-beat pressure-volume loops were recorded during inferior vena cava occlusion in 39 HFpEF patients and 20 controls during handgrip and atrial pacing. We measured the contribution of relaxation, elastic recoil, and stiffness to instantaneous diastolic pressure using a novel processing method. Fibrosis was quantified from endomyo-cardial biopsies. HFpEF patients showed higher diastolic pressures and stiffness constant than controls (P < 0.05 for all). As opposed to controls, all intrinsic global diastolic properties were sensitive to acute changes in systolic pressure in the HFpEF group. In fact, the stiffness constant increased by more than 50% during handgrip in HFpEF patients (P < 0.05), tightly related to changes in systolic pressure (fixed-effect = 0.26mm Hg per mm Hg [95% CI = 0.15-0.37]; P < 0.0001). Incomplete relaxation contributed to increasing pressure before atrial contraction, but changes in end-diastolic pressure was mostly caused by the increase in stiffness. The degree of pressure-sensitivity of stiffness correlated with myocardial collagen volume and crosslinking (R = 0.40 to 0.82 for all). Conclusion Acute chamber stiffening is the main mechanism responsible for rising late-diastolic pressures when HFpEF patients undergo hypertension transients. This stiffening behaviour is related to impaired dynamic systolic-diastolic interactions and correlates with matrix remodelling. Ventricular-vascular relationships are a promising target in HFpEF and should be taken into account when assessing diastolic function.