AbstractDysprosium‐165–ferric hydroxide macroaggregates (165Dy‐FHMA) was used as an agent of radiation synovectomy in an antigen‐induced arthritis model in New Zealand white rabbits. Animals were killed up to 6 months after treatment.165Dy‐FHMA was found to have a potent but temporary antiinflammatory effect on synovium for up to 3 months after treatment. Treated knees also showed significant preservation of articular cartilage architecture and proteoglycan content compared with untreated controls, but only during the first 3 months after treatment. In animals killed 3 and 6 months after treatment there were only minimal differences between the treated and untreated knees, indicating that the antiinflammatory effects on synovial tissue and articular cartilage preservation were not