This unusual clinicopathologic entity was first recognized by Castleman. Onset is usually in adolescence or early adulthood, occasionally childhood. A single, large mass with no tendency to compress develops, usually in the mediastinum, occasionally at other lymph node sites (superficial, retroperitoneal, abdominal), and exceptionally in extranodal locations. A variety of systemic manifestations may be found, including fever, anemia, hypergammaglobulinemia, and a broad spectrum of inflammatory and autoimmune disorders, presumably associated with specific humoral factors produced by the tumor. Histologic examination shows the characteristic features of angiofollicular lymph node hyperplasia. Two variants have been described, i.e., the plasma cell type (active stage) associated with systemic manifestations, and the hyaline vascular type (quiescent stage). Surgical removal of the tumor ensures permanent (local and systemic) recovery in the overwhelming majority of cases. A role of microorganisms in the etiology of this condition has been suggested. Identical histologic changes can be found in adults with immune dysfunction syndromes. The etiology, pathogenesis and nosology of this very benign disease are still poorly known.