Project Summary Subchondral bone marrow edema lesions are a characteristic magnetic resonance imaging (MRI) finding of sacroiliitis in patients with axial spondyloarthritis (SpA). The histopathological equivalent of bone marrow edema in axial SpA is unknown. Based on studies in rheumatoid arthritis, we hypothesize that the bone marrow edema lesions in axial SpA represent inflammatory infiltrates. In this pilot study, patients with axial SpA and healthy controls will undergo MRI-guided bone marrow biopsy to determine the histopathology of bone marrow edema lesions detected on MRI and identify major immune cell subsets present in these lesions. The results from these experiments will determine feasibility and guide larger scale investigations into the pathogenesis of axial SpA in the future. Understanding SpA pathogenesis at the level of the diseased tissue is a key requirement for the development of strategies to cure or prevent this disease.