Summary The malignant giant cell tumour of bone is described here as the malignant growth which contains many foreign body giant cells (osteoclasts). This tumour is at first localized to one part of a bone, radiologically shows characteristic trabeculation and histologically shows a structure closely resembling that of the benign giant cell tumour. The features distinguishing it from the benign giant cell tumour are the relatively uniform stroma and the presence of mitotic figures. It is the less malignant form of this tumour, and the lack of clear segregation of it from the benign giant cell tumour that is responsible for much of the confusion regarding the benign tumour. Difficulties of histological diagnosis are due to foreign body giant cells and a superadded pleomorphic stroma occurring in malignant tumours as the result of haemorrhage and infection. The benign giant cell tumour does not fulfil the criterion of a neoplasm, namely that of progressive growth, and thus its distinction from the true neoplastic form is extremely important.