A case of malignant giant cell tumor of the tendon sheath of the right hip, which developed in a 72-year-old Japanese woman, is described. The tumor exhibited histological similarities to a benign giant cell tumor of the tendon sheath (localized nodular tenosynovitis). The resected tumor, measuring 9 x 9 x 11 cm, was located in the adductor muscle and invaded the proximal femur and acetabulum. The nodule was encapsulated with a thin membrane which was soft and gelatinous in consistency and varied in color from yellow to brown. The synovium of the hip joint was normal. The primary lesion was composed of plump polyhedral and spindle-shaped cells. The nuclei were large, irregular and hyperchromatic, and contained prominent nucleoli. A moderated number of multinucleated giant cells was scattered throughout the lesion. There was little stromal collagen. In the majority of the specimens, pseudoglandular or alveolar spaces were predominant. An ultrastructural study demonstrated three cell types: fibroblast-like, histiocyte-like and an intermediate. The patient underwent reconstructive surgery with a Dacron fabric-enveloped alumina ceramic pelvic prosthesis and total hip components after resection of the primary lesion. Unfortunately, because of a local recurrence, a hemipelvectomy was required 10 months after the initial operation. At that time the intestines were involved with the recurrent tumor, and the patient subsequently died of perforative peritonitis. An autopsy revealed distant metastases to the right pelvis, urinary bladder, right ureter, ilium, mesenterium and lungs.