Post-infection chronic fatigue syndromes, such as myalgic encephalomyelitis/chronicfatigue syndrome (ME/CFS) and post-COVID-19 condition (Long Covid), are conditionsprimarily characterized by debilitating fatigue. This fatigue can range from mild, wherepatients are still able to participate in some social activities (e.g., school, work), tomoderate and severe, where sufferers are predominantly homebound and bedridden. As aresult, ME/CFS and Long Covid not only negatively impact the quality of life of affectedindividuals and their caregivers but also represent a substantial and often silent burdenon healthcare systems worldwide, including Austria. This is primarily because most casesremain undiagnosed due to the lack of standardized clinical assessments and diagnosticmarkers. Endothelial dysfunction, which is well known to affect blood flow, oxygen andnutrient delivery, and waste removal in the body, has been described as one of the keyfactors behind the symptoms experienced by ME/CFS and Long Covid patients. However, themechanisms that might explain the development of endothelial dysfunction remain largelyunexplored. Therefore, this project aims to evaluate key biological aspects related tothe function of endothelial cells - a layer of cells lining blood vessels - using plasmasamples from an Austrian cohort of ME/CFS and Long Covid patients. We expect that thefindings from our study will provide new insights to better understand endothelialdysfunction in post-infection chronic fatigue syndromes, leading to improved patientstratification and tailored treatment alternatives.