A randomized cooperative double-blind trial of oral L-histidine for the treatment of rheumatoid arthritis was carried out. Patients were treated with either L-histidine 4.5 g daily, or placebo, for 30 weeks. None of the clinical measurements showed an advantage of histidine over placebo. A small decrease in rheumatoid factor titer and a small increase in hematocrit were found only in the histidine group. There was suggestive evidence of a beneficial effect of histidine in patients with more active and prolonged disease, based upon subjective doubld-blind evaluations by physicians and patients. No adverse effects of histidine therapy were noted. Histidine cannot be advocated as a therapeutic agent in rheumatoid arthritis, but further studies in certain groups of patients seem justified.