BACKGROUND: Cardiac transplantation remains the most effective treatment for patients with end-stage heart failure, but its use is limited by donor organ shortage. Expanding donor acceptance criteria may increase the availability of transplantable hearts. Left ventricular dysfunction due to neurogenic stunned myocardium is common among organ donors and is characterised by rapid functional and structural recovery. The IMPROVED Heart (IMPact of donor heart function on Recipient Outcomes: a prospectiVE observational study to increase the utilisation of Donor Hearts) study aims to increase the number of heart transplantations by systematically evaluating and using donor hearts with temporarily impaired function. METHODS AND ANALYSIS: IMPROVED Heart is a prospective, multicentre observational study. The primary objective is to determine whether recipients of donor hearts with regional wall motion abnormalities and/or mild to moderate global left ventricular dysfunction have outcomes comparable to recipients of hearts with normal function. Cardiac function in potential heart donors is assessed systematically using repeated echocardiography in addition to cardiac biomarkers and clinical data. Heart transplant recipients are managed according to routine clinical practice. Based on a non-inferiority power calculation, 445 transplanted hearts-including at least 89 with impaired function-are required. The study started enrolling in 2022. ETHICS AND DISSEMINATION: The study was approved by the Swedish Ethical Review Authority in March 2020 (Dnr 2019-06229). Study findings will be presented at scientific meetings and published in international peer-reviewed journals. TRIAL REGISTRATION NUMBER: NCT04393181.