The Stoke Index is a validated composite algorithm that has been designed to give a global measure of disease activity in rheumatoid arthritis (RA). The use of this single measure of disease activity in RA simplifies the critical evaluation of drug therapy. 368 patients with RA of varying duration and severity, entered into comparative drug trials between 1980 and 1987, had the algorithm calculated four weeks prior to therapy, at the start of treatment, and bi-monthly to six months. The index score was significantly improved by drugs with known slow acting anti-rheumatic drug (SAARD) activity and improvement could be seen as early as two months after the beginning of treatment. Non-steroidal anti-inflammatory drugs (NSAIDs) did not improve the score. The index differentiates between treatments in patients with minor or major disease activity. We conclude that this composite index of disease activity provides a sensitive, meaningful measure for the evaluation of therapy in RA.