In an effort to understand the prognostic features that may influence the probability of a patient's continuing to take methotrexate (MTX) over time, we studied 152 rheumatoid arthritis patients treated with MTX between 1981 and 1986. The overall probability of continuing to take MTX was 71.2% at 1 year, 55.5% at 3 years, 50% at 5 years, and 49% at 6 years. By univariate analysis, patients who started MTX therapy later in the study, American blacks, younger patients, those with less severe disease, and those with less frequent or less severe toxic events appeared to have a better probability of continuing the drug therapy. When these parameters were evaluated by multivariate analysis, only the time when MTX was started and the occurrence of toxic effects independently influenced the probability of continuing MTX. Thus, by current practice standards, toxic effects emerge as the main reason for MTX discontinuation.