Two female patients with the typical clinical and electrophysiological features of the stiff-man syndrome, both responded to steroid treatment. ACTH infusion produced an immediate clinical relief of muscle contracture and cramps, with parallel marked reduction of the EMG pattern of continuous spontaneous activity in agonist and antagonist muscles. Apart from this effect, a more delayed response to oral prednisone was observed in both cases and steroid-dependence in one of them, who also exhibited instrumental and laboratory findings suggesting an inflammatory process. These data lead us to consider a possible dysimmune pathogenesis of some cases with the stiff-man syndrome.