This report presents four cases of presumed malignant transformation within intraosseous lipomas, one occurring in a stage I lesion and three in stage III lesions, according to my classification. These lesions demonstrated radiologic and histologic features of benign lipomas together with histologic fields of either malignant fibrous histiocytoma or liposarcoma. In two of the three patients who were followed up, the tumors were fatal despite amputation. Malignant transformation of a lipoma should be suspected when rapid bone destruction is seen in a stage I radiolucent lipoma. Malignant transformation of stage III lipomas could be mistaken for malignant transformation within bone infarcts.