A clinical, angiographic, pathologic-anatomic, and cytologic study of 12 cases of malignant giant cell tumor of soft tissues is presented. Preoperative angiography was performed in five cases, one of which was also studied by microangiography. Aspiration cytologic examination was performed in five cases and electron microscopy in one case. In nine cases the tumors were deeply situated and in three they were subcutaneous. In six cases the tumors were located within the lower extremity, in four cases in the upper extremity and shoulder, in one case retroperitoneally and in one case in the neck. Except for one case, the follow-up period was 1–14 years. Seven of the patients had died from metastasizing tumor disease at the time of follow-up. High vascularity and rapid circulation characterized the tumors at preoperative angiography. Conventional histology, aspiration cytology, and electron microscopy disclosed histiocyte-like cells, fibroblast-like cells, and multinucleated giant cells of both osteoclast-like benign and pleomorphic malignant type, in varying proportions. The histiocyte-like cells and the giant cells exhibited phagocytic activity at these examinations. The finding of phagocytosing pleomorphic malignant cells and giant cells of osteoclast type in aspiration cytologic material strongly favors the diagnosis of malignant giant cell tumor of soft tissues.