A case of so-called "benign giant cell tumor of bone" with an incidental histologic finding of intratumor vascular invasion is reported. The mechanism and biology of metastasis are briefly discussed. For the purposes of this presentation, the mechanism of metastasis is divided into two types--active and passive. An active type of metastasis indicates malignancy, whereas a passive type denotes a benign process. The malignant features of the conventional or typical giant cell tumor of bone are demonstrated. It is proposed that this neoplasm be labeled malignant despite its seemingly benign histologic appearance. Relative to the degree of malignancy, a lesion of this nature may be classified as either a low or a high grade type of malignant giant cell tumor.