Thirteen patients with moderately severe myasthenia gravis participated in a double-blind study using either 100 mg of prednisone or an equivalen numbder of placebo tablets on alternate days. Anticholinesterase therapy was continued on a demand basis. At the end of 6 months the code was broken. Seven patients were on placebo and three of these had improved to such a degree that steroid therapy was not indicated. Four of these patients ultimately were started on prednisone and improved. Of the six patients on prednisone, three showed no improvement and three were improved. At the end of 2 years, the seven patients still taking prednisone were on maintenance dosage of this drug. Three of this group had experienced relapses when the dosage was cut to 15 to 25 mg on alternate days, and they again improved when the prednisone dosage was increased. No statistical evaluation is possible because of the small number of patients. It can be stated that seven patients improved with steroid therapy. Conversely, not all patients treated with alternate-day prednisone improved. Finally, any evaluation of treatment of myasthenia gravis must take into consideration the potential for spontaneous improvement, as demonstrated by three of the patients treated with placebo.