Local deep microwave hyperthermia (LDMWH) may be considered as a therapeutic tool for joint diseases because it has the advantage of heating the target organ e.g. synovium, while sparing the adjacent tissues, as demonstrated in animals. The effect of this new microwave device has been evaluated on seven rheumatoid arthritis patients with knee effusion. The hyperthermia apparatus consists of a 915 MHz power source with a cooling system to the skin. LDMWH was operated for one hour, twice a week for two weeks. Intra-articular knee temperature reached the level of 40.6 +/- 1.1 degrees C within 15 minutes, raised to 41.2 +/- 0.7 degrees C after 30 minutes and to 41.3 +/- 0.9 degrees C at one hour. The skin temperature over the heated knee joints remained at 24.3 +/- 1.1 degrees C during treatment. All patients noted a sensation of warmth in the treated knee. Aspiration of synovial fluid was performed before and immediately after each treatment. Walking time was improved (p = 0.04) and significant decrease in pain (p = 0.01) was noted following treatment. Synovial fluid leukocyte count and prostaglandin E2 level, knee circumference and range of motion did not change. Severity of pain, walking time, knee circumference and range of motion remained stable in the six weeks following the last hyperthermia application. No adverse reaction could be observed. These results suggest LDMWH to be safe and successful as an adjuvant treatment of chronic inflammatory joint effusions.