Giant cell tumors (GCTs) of bone, also called osteoclastomas, complicate Paget bone disease (PBD), though infrequently. Giant cell reparative granulomas (GCRGs), which are histologically similar to GCTs, also occur rarely in pagetic patients. A 45-yr-old black woman with neurofibromatosis, type I, and polyostotic PBD developed slowly-growing masses in the right posterior iliac and left upper parasacral regions. She had multiple cutaneous neurofibromas and cafe-au-lait spots. Serum alkaline phosphatase activity and urine hydroxyproline levels were elevated. Skeletal radiographs and bone scintigraphy showed changes of widespread PBD. Computerized tomography and magnetic resonance imaging (MRI) delineated masses in the right gluteal and the left lower lumbar paraspinal regions. Five additional smaller masses were found in the abdomen and in the pelvis. Biopsy of the right gluteal mass revealed a GCT. However, we found that this lesion had several histologic features distinct from those of giant cell reparative granulomas or GCT. In our patient's tumor, the huge polykaryons, like osteoclasts, expressed abundant tartrate-resistant acid phosphatase activity, whereas those of GCRG lack this enzyme. Although the polykaryons in conventional GCTs and GCTs in PBD express tartrate-resistant acid phosphatase activity, the location of these tumors in bone differs from the extraskeletal masses encountered in our patient. Furthermore, the larger size of the polykaryons and the greater number of nuclei in our patient's GCT differ from conventional GCTs, but not GCTs in PBD. Her extraskeletal osteoclastema rapidly shrunk to one third its original size during 2 weeks of oral dexamethasone treatment. Significant clinical improvement lasted about 5 months before additional courses of dexamethasone therapy were necessary. Injections of synthetic salmon calcitonin alone did not affect the tumor's size. Thus, PBD can be complicated by extraskeletal tumors that seem to contain osteoclasts and are responsive to dexamethasone treatment.