Abstract Background The prevalence of heart failure with preserved ejection fraction (HFpEF) increases with the aging of our population and shares risk factors with cancer. Furthermore, HFpEF conveys a worse prognosis if it remains unrecognized and untreated. However, it is not considered as a surrogate marker in the baseline risk stratificatoin of patients that will undergo potentially cardiotoxic cancer treatments. Moreover, the definition of cardiotoxicity focuses mainly on systolic dysfunction, based on left ventricular ejection fraction and global longitudinal strain changes. Data on the prevalence of new-onset HFpEF during cancer treatment are limited. Purpose We evaluated the prevalence of HFpEF in cancer patients followed in our cardio-oncology clinic, with new cancer therapy-related cardiovascular toxicity (CTR-CVT) events in patients with known HFpEF prior to cancer therapy. We also assessed the prevalence of patients diagnosed with HFpEF while undergoing cancer therapy. Methods A total of 630 cancer patients referred for either baseline evaluation, on-treatment surveillance, long-term follow-up or new complaints suspect for cardiovascular disease were included in this real-world observational study, with a mean follow-up duration of 507 days. They underwent comprehensive workup in accordance with the ESC guidelines, including clinical assessment, electrocardiogram, echocardiography and biomarker testing. Results From the total population, 36 (5.7%) were diagnosed with HFpEF prior to cancer treatment initiation. Among these, 18 (50%) developed CTR-CVT during follow-up, mainly heart failure events (cancer-therapy related cardiac dysfunction or HFpEF decompensation). Independent of baseline characteristics, 233 patients (37.0%) developed a heart failure event during or after cancer treatment. Of these, 92 patients (14.6% of the total population, 39.5% of all HF events on treatment) developed a HFpEF event during or after cancer treatment. Of these HFpEF events, 42 patients (45.7%) were known with pre-existing cardiovascular disease, of whom only 14 (15.2%) with pre-existing HFpEF. In 50 patients (54.3%) the HFpEF diagnosis was made de novo during (or after) cancer treatment. Patients developing HFpEF were mostly female (n=54, 58.7%) and relatively young (mean age 62.0 years). Most HFpEF events occurred in breast cancer patients (n=32, 34.7%), prostate cancer patients (n=13, 14.1%) and multiple myeloma patients (n=9, 9.8%). Conclusions HFpEF is frequently overlooked, but conveys a high risk of developing CTR-CVT and should therefore be taken into account in the baseline risk stratification. Furthermore, HFpEF is responsible for an important morbidity burden during or after cancer cardiotoxic treatments, even in patients without prior diagnosis of HFpEF at baseline evaluation. Further studies are needed to evaluate if this population may benefit from specific management and treatment strategies in order to improve prognosis.Prevalence of HFpEF in cancer patients