This report reviews the clinical and pathologic features of 423 examples of sinus histiocytosis with massive lymphadenopathy (SHML) entered in a case registry, with special emphasis on extranodal manifestations of the disease. The most common sites of extranodal involvement in this population are skin, upper respiratory tract, and bone. However, SHML also can occur in a variety of other sites, including the genitourinary system, lower respiratory tract, oral cavity, and soft tissues. Involvement of kidney, lower respiratory tract, or liver was found to be a poor prognostic sign, and patients with associated immunologic disease often fared poorly. In general, prognosis has been found to correlate both with the number of nodal groups and with the number of extranodal systems involved by SHML. A complete bibliography of publications describing patients with SHML is included, and illustrations of the clinical, histopathologic, and radiographic features of the disease are provided.