Twenty-five patients have completed a mean of 53 months of treatment with methotrexate (MTX) as part of a prospective study of the long-term safety and efficacy of the drug. Since the time of the last report (at a mean of 29 months), the mean dosage of MTX has increased from 12.4 mg/week to 14.6 mg/week, whereas the mean prednisone dosage has decreased from a baseline of 7.1 mg/day to 1.9 mg/day. A significant improvement from baseline in all clinical parameters tested was maintained, and response to therapy did not vary significantly between the assessment at 29 months and that at 53 months. Toxic reactions were as common during months 30-53 as during the first 29 months of the study, with patterns of toxic reactions remaining consistent within each patient. Radiologic evidence of disease progression was not seen before 24 months of MTX treatment, but after this time, it was observed in some patients. We conclude that many clinical features of long-term MTX therapy are distinctly different from what might have been expected after the short-term trials.