An 80-year-old man was noted to be bradycardic while being transferred to the United Kingdom following neck of femur fracture abroad. He had a 3-month history of weight loss, pruritus, and bone pain. Echocardiogram and cardiac magnetic resonance imaging showed a concentric right atrial mass causing luminal narrowing (panel A, arrow), pericardial effusion, and circumferential infiltration of descending aorta and arch adventitia. Bilateral perinephric soft tissue infiltration was found on computed tomography scan of his abdomen (panel B). Skeletal radiographs demonstrated bilateral patchy sclerosis of the metaphyses of the distal femora (panel C), as well as generalized increased uptake of long bones on nuclear medicine bone scan (panel D). Biopsy of the perinephric tissue revealed infiltration by lipid-laden histiocytes (panel E, arrows; original magnification ×100, hematoxylin and eosin stain), which were strongly positive for CD68 and factor 13a, but negative for S100 and CD1a, confirming the diagnosis of Erdheim-Chester disease. BRAF V600E mutational testing, using pyrosequencing on the perinephric and bone biopsy tissues, was negative.Erdheim-Chester disease is a rare multisystem non–Langerhans cell histiocytic disorder. Symmetrical long bone osteosclerosis sparing the epiphyses and axial flat bones is the commonest manifestation of the disease. Cardiac involvement is present in about a third of cases. However, a large atrial mass with impending superior vena cava obstruction is relatively uncommon.