Synoviorthesis has the same indications as surgical synovectomy. But it is easy to perform, can be repeated and does not leave any scars. It should be tied first. The choice of the product to be injected depends on the joint, the number of injections and the age of the patient. In all cases a long-acting local steroid should be tried first. In combination, osmic acid seems to potentiate the effect of the steroid. If osmium tetroxide damages the cartilage, it has been demonstrated that the damage is limited and superficial and as far as we know without clinical consequence. The fear of side-effects has limited the use of synoviortheses in the United States. Sledge et al. (1977), in animals, have used a new rare earth isotope, dysprosium-165, a beta-emitter with a maximum tissue penetrance of 5.7 mm and the advantage of a very short half-life of 140 minutes. In so far as systemic therapy is only partly successful there is still a place for chemical and radiation synovectomy in rheumatic diseases.