Comparison of the ability to detect a flare of joint inflammation provoked by withdrawal of non-steroidal anti-inflammatory drugs from 10 patients with rheumatoid arthritis was made between 28 computer generated articular indices. The results suggest that this method can be used to compare the sensitivity of articular indices. Identifying joint inflammation by the simultaneous presence of tenderness and swelling and weighting for joint size produced the most sensitive indices. Identifying inflamed joints by swelling alone, or swelling and/or tenderness, grading for the severity of the signs, or selecting a restricted range of joints provided no advantages.