Though heat has been used for many years for the treatment of painful joints such therapy rarely increases the intra-articular temperature significantly. This pilot study investigated the effect of intra-articular hyperthermia induced by microwaves from a combined heating/cooling instrument on zymosan-induced synovitis in the rabbit. There was histological evidence not only of a reduction in the intensity of the inflammatory response but also in its type with the absence of giant cell formation. However, hyperthermia was not totally without effect on the synovium of control joints and further studies are required to investigate the relevance of this effect.