OBJECTIVE: To determine the effect of two different weekly doses of folic acid on the toxicity and efficacy of low-dose methotrexate therapy for rheumatoid arthritis. DESIGN: Randomized, double-blind, placebo-controlled study. PATIENTS: 79 persons between 19 and 78 years of age who fulfilled the American Rheumatism Association's criteria for rheumatoid arthritis. INTERVENTION: Participants were randomly assigned to visually identical placebo or to 5 mg or 27.5 mg of folic acid each week. MEASUREMENTS: Duration, intensity, and clinical severity of toxic events; efficacy (indices of joint tenderness and swelling and grip strength); plasma and erythrocyte folate levels; and other laboratory variables. RESULTS: Folic acid supplementation at either dose did not affect the efficacy of methotrexate therapy as judged by joint indices and patient and physician assessments of disease. Patients given folic acid supplements had lower toxicity scores than did participants given placebo (P < or = 0.001). Low blood folate levels and increased mean corpuscular volumes were associated with substantial methotrexate toxicity, whereas daily dietary intakes of more than 900 nmol (400 micrograms) of folic acid were associated with little methotrexate toxicity. CONCLUSIONS: Folic acid, an inexpensive vitamin, is safe in a broad range of doses and protects patients with rheumatoid arthritis who are taking methotrexate from toxicity while preserving the efficacy of methotrexate.