Wereport a patient with polymyalgia rheumatica (PMR) accompanied by an increased Rheumatoid Arthritis Hemagglutinin Test (RAHA)titer and interleukin-6 level in the synovial fluid. A 60-year-old female was admitted because of polymyalgia, a body temperature of 39.2°C, and an erythrocyte sedimentation rate increased to 94 mm/h. Since a muscle biopsy failed to show a specific finding, she was diagnosed as PMR.The titer of RAHAand the interleukin-6 level were increased only in the synovial fluid; prednisolone treatment decreased both. The present case raised the possibility that a similar mechanism in rheumatoid arthritis may involve the development of synovitis in PMR. (Internal Medicine 32: 484-486, 1993)