The anticonvulsant drug phenytoin has several interesting immunological properties which could theoretically be of benefit in the treatment of rheumatoid arthritis. For this reason a pilot study has been carried out on 11 patients with active classical or definite disease. Seven patients completed a 20 week course of treatment and showed continuous improvement at 12 and 20 weeks and some deterioration eight weeks after the drug was stopped. Laboratory and clinical measurements of disease activity responded favourably during the treatment period, suggesting that phenytoin may have second line activity.