THE PRESENCE of multinucleated giant cells in a variety of bone lesions has led to inclusion of certain neoplastic, inflammatory, developmental, and metabolic disorders of bone under the diagnosis of giant-cell tumor or giant-cell tumor variant. In 1940, Jaffe, Lichtenstein, and Portis (1) offered a specific definition of giant-cell tumor which has gained wide, though not universal, acceptance. They considered it a distinctive neoplasm apparently arising in the nonosteoblastic connective tissue, composed of a vascularized network of spindle-shaped or ovoid stromal cells interspersed with multinuclear giant cells (which they regarded as syncytial stromal cells). In subsequent publications Jaffe and Lichtenstein (2–6) identified as distinct clinical and pathologic entities many of the lesions formerly classed as giant-cell tumor variants. Accurate identification of lesions in this category is of considerable practical importance. Most of the socalled giant-cell tumor variants are fundamentally benign lesion...