INTRODUCTION: Left ventricular diastolic dysfunction (LVDD) is a common condition in both sexes that may deteriorate into heart failure (HF) with preserved ejection fraction (pEF), although this seems to happen more often in women than in men. Both LVDD and HFpEF often go unrecognised, necessitating the discovery of biomarkers that aid both the identification of individuals with LVDD at risk of developing HF and identification of individuals most likely to benefit from treatment. METHODS AND ANALYSIS: HELPFul is an ongoing case-cohort study at a Dutch cardiology outpatient clinic enrolling patients aged 45 years and older without history of cardiovascular disease, who were referred by the general practitioner for cardiac evaluation. We included a random sample of patients and enriched the cohort with cases (defined as an E/e' ≥8 measured with echocardiography). Information about medical history, cardiovascular risk factors, electrocardiography, echocardiography, exercise test performance, common carotid intima-media thickness measurement and standard cardiovascular biomarkers was obtained from the routine care data collected by the cardiology outpatient clinic. Study procedure consists of extensive venous blood collection for biobanking and additional standardised questionnaires. Follow-up will consist of standardised questionnaires by mail and linkage to regional and national registries. We will perform cardiac magnetic resonance imaging and coronary CT angiography in a subgroup of patients to investigate the extent of macrovascular and microvascular coronary disease. ETHICS AND DISSEMINATION: The study protocol was approved by the Institutional Review Board of the University Medical Center Utrecht. Results will be disseminated through national and international conferences and in peer-reviewed journals in cardiovascular disease. TRIAL REGISTRATION: NTR6016;Pre-results.