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Background Heart failure with preserved ejection fraction (HFpEF) is characterised by systemic congestion and elevated left ventricular filling pressures. Pulmonary transit time (PTT) measured by cardiovascular magnetic resonance (CMR) provides a non-invasive, integrated assessment of cardiopulmonary haemodynamics, but its prognostic value in patients with HFpEF remains uncertain. We aimed to determine whether prolonged PTT independently predicts adverse clinical outcomes in HFpEF.Methods Adult patients with HFpEF were prospectively recruited and underwent comprehensive phenotyping including blood sampling, 12-lead electrocardiography, 6-min walk testing, echocardiography and multiparametric CMR ( NCT03050593). PTT was derived from rest first-pass perfusion imaging and normalised to cardiac cycle length. An abnormal PTT threshold was defined as >95th percentile of asymptomatic controls. The primary endpoint was a composite of heart failure hospitalisation or all-cause mortality. Multivariable Cox proportional hazard regression (HR) models were used to investigate associations with clinical outcome.Results One-hundred and eighteen HFpEF patients were studied (median follow-up 7.7 years). Eighty-one composite events occurred (50 heart failure hospitalisations and 31 deaths). Prolonged PTT independently predicted the composite outcome across four separate models: clinical (HR 2.222 (1.222, 4.041), p=0.009), blood biomarker (HR 1.776 (1.022, 3.087), p=0.042), imaging (HR 2.108 (1.115, 3.983), p=0.020) and a combined model incorporating the strongest markers (HR 2.989 (1.612, 5.543), p<0.001).Conclusion Prolonged PTT is an independent predictor of death or heart failure hospitalisation in HFpEF. As an easily obtainable and integrative CMR biomarker of cardiopulmonary haemodynamics, PTT may improve risk stratification in HFpEF, although its clinical application requires further investigation.