Rationale: The diagnosis and pathogenesis of long COVID remains unknown. We havepreviously shown that [68Ga]FAPI Positron Emission Tomography-Computed Tomography(PET/CT) imaging shows potential for diagnosis and molecular understanding of thissyndrome. We have previously shown that fibroblast activation protein (FAP) can be imagedin the lung, muscle and nasopharynx of long COVID patients (with dyspnea and fatigue).However, these preliminary data are derived from a selective group of patients with longCOVID after critical COVID-19. We aim to explore the generalizability of these findingsin patients with long COVID with dyspnea and fatigue, irrespective of the severity oftheir acute SARS-CoV-2 infection.Primary objective: To assess if pulmonary fibroblast activity, measured by [68Ga]FAPI-46PET/CT, is higher in patients with current long COVID dyspnea and fatigue compared topatients with resolved complaints.Study design: This is a ZonMw funded single centre prospective observational cohort studyof long COVID-19 patients with dyspnea and fatigue.Study population: We will recruit 60 adult long COVID patients (aged >20 years) of which30 have complaints of dyspnea and fatigue and compare them to 30 patients with resolvedcomplaints and healthy controls.Main study parameters/endpoints: The primary endpoint is FAP expression in the lungmeasured by [68Ga]FAPI-46 PET/CT. Secondary endpoints are the expression of FAP in othertissues (muscle) and the relation between FAP and inflammation and remodelling biomarkersin various biological samples (e.g. serum/nasal epithelium).Study procedures: In a single visit day the following data and samples will be collected:questionnaires, a lung function test, 6-minute walking test, blood samples, nose swabs,[68Ga]FAPI PET/CT scan and HRCT scan. When increased [68Ga]FAPI uptake is measured in themuscles a muscle biopsy will be performed as well.