1. The literature on pigmented villonodular synovitis has been reviewed and a series of eighty additional cases is reported.

2. The condition usually presents either as a nodule in a finger or knee, or as a diffuse lesion in a knee. The lesions, although benign, sometimes erode or invade the tissue of adjacent bones.

3. Distinction from malignant synovioma can be made on the basis of the macroscopic appearance of the lesion at operation (relationship to joints or tendon sheaths: villonodular appearance: pigmentation), and by histological examination.

4. Treatment of the nodular form by excision is satisfactory but extensive synovectomy for diffuse lesions of the knee gives poor results.

5. The etiology of pigmented villonodular synovitis is unknown, but it appears to be a self-limiting process, possibly inflammatory in nature.

1. One hundred and seventy-nine cases of primary malignant bone tumour and giant-cell tumour seen at the Middlesex Hospital since 1925 are reviewed. Tumours arising from non-skeletal tissues in bone have been excluded.

2. The following histological classification is used. Osteosarcoma (osteoblast sarcoma): This tumour is not synonymous with osteogenic (bone-forming) sarcoma. The essential feature is the formation of osteoid tissue by malignant osteoblasts, with no intermediate matrix of cartilage or fibrous tissue. It is the most malignant bone tumour and only four of the thirty-two patients survived three years. Chondrosarcoma: These tumours are composed of cartilage, and some show secondary ossification. The behaviour of this group is related to the degree of cartilaginous differentiation. In general, compared with the osteosarcoma, it is of low-grade malignancy. More than half of the sixty-eight patients survived four years. Fibrosarcoma: The essential feature of this tumour is the production of collagen by malignant fibroblastic tumour cells. Tumours of this type invading the medullary cavity have an average prognosis between that of an osteosarcoma and a chondrosarcoma. Nine of the thirty-four patients survived three years. Spindle-cell sarcoma: These tumours are composed of spindle cells which produce no diagnostic matrix. In spite of the lack of differentiation the outlook is not hopeless. Six of the eleven patients survived for five years or more. Giant-cell tumour: This tumour is composed of a cellular stroma with diagnostic giant cells resembling osteoclasts. It is by no means a benign lesion, for half the tumours recurred after treatment and a quarter of the patients died with metastases.

3. The subdivision of primary malignant skeletal tumours into groups according to the histological pattern appears to be reflected in the behaviour of the individual tumours after treatment. The prognosis of each group has been stated in the appropriate sections.