A persistently normal erythrocyte sedimentation rate in the presence of active synovitis was found in 17 out of 500 patients with rheumatoid arthritis. These were compared with 17 age- and sex-matched random rheumatoid arthritis controls. The patients were mostly female (15/17) and seronegative (12/17). Other acute phase reactants were measured in some cases and were also normal. Athough erosion and deformity occurred in more than half, significant involvement was restricted to the small, peripheral joints. The limited mass of inflamed synovial tissue may account for the absence of an acute phase response.