A composite index for estimating improvement in individual rheumatoid arthritis (RA) patients during trials of slow-acting, disease-modifying antirheumatic drugs (DMARDs) was developed by analyzing the responses of 130 placebo-treated participants in Cooperative Systematic Studies of Rheumatic Diseases studies. If responses in 4 of 6 selected measures were required for improvement (by ⩾20% for morning stiffness, Westergren erythrocyte sedimentation rate, joint pain/tenderness score, and joint swelling score, and by ⩾2 grades on a 5-grade scale, or from grade 2 to grade 1 for patient's and physician's overall assessments of current disease severity), few placebo-treated patients qualified as improved, whereas significantly more DMARD-treated patients demonstrated improvement. The proposed index appears to be useful in estimating the probability that an RA patient will improve if taking a placebo during a DMARD trial, and may be a useful tool for analysis of DMARD studies.