Abstract
Rosai-Dorfman disease is a rare, idiopathic disorder of histiocyte proliferation. We describe a case of a 59-year-old woman who presented with heart failure symptoms from a large pericardial mass causing constrictive pericarditis. Pathologic findings including immunohistochemistry suggested the diagnosis. She was treated with corticosteroids and improved clinically. (Level of Difficulty: Advanced.).