The efficacy of phenytoin in the treatment of the symptoms of diabetic symmetrical polyneuropathy has been tested in a double-blind, crossover study. Symptoms were evaluated daily by linear analogue self-assessment, while control of blood glucose and plasma phenytoin level were monitored on a weekly basis. There was no significant improvement in symptoms on phenytoin, whether analyzed in aggregate, as pain alone, or on the last 3 days of each treatment week. Blood glucose, however, was elevated in diabetic patients taking phenytoin, and the incidence of undesirable side effects was increased. It is concluded that phenytoin has no role in the treatment of diabetic symmetrical polyneuropathy.