Malignant giant cell tumor is a confusing term that in the past has been used to describe different types of giant cell-rich tumors. We try to clarify this term in this report. We consider two types of malignancy in giant cell tumor of bone: "primary" when it arises in juxtaposition to a benign giant cell tumor and 'secondary' when it arises at the site of a previously treated giant cell tumor. Here we present a case of primary malignancy in giant cell tumor that was initially not recognized as a malignancy. On radiography and histology of frozen sections the lesion had the appearance of a conventional giant cell tumor of bone. After curettage, the permanent histology slides showed areas of highly malignant osteosarcoma juxtaposed to areas of benign giant cell tumor. The patient was treated with chemotherapy and wide resection of the tumor.