A double-blind controlled trial was performed on 6 patients affected with Steinert's disease in order to evaluate the efficacy of two different dosages of diphenylhydantoin (PHT, 200 and 300 mg/day) and carbamazepine (CBZ, 600 and 800 mg/day) on the myotonic afterdischarge. Both dosages of PHT and CBZ induced a significant improvement of myotonia. For PHT a trend towards decreased efficacy is pointed out at toxic or at high dosages.