Giant cell tumors of bone are sometimes locally aggressive and may metastasize, although uncommonly. We attempted to identify associations of clinical and histopathologic parameters with metastasis, the longterm outcome with metastases, and the best treatment. We identified distant metastases in 24 of 470 patients with giant cell tumors during a 20-year period. The median age of these 24 patients at presentation was 26 years (range, 16-76 years), and the male:female ratio was 1.6:1, with no predilection for primary site. Metastasis occurred at a mean of 2 years (range, 4 months-11 years) after initial diagnosis. Sites for distant metastases were the lung (21 of 24 patients), scalp, calf muscle, and regional lymph nodes. The 24 patients had a mean followup of 3.5 years (range, 0-16 years