A 57-year-old woman developed muscular stiffness and painful cramps, which were relieved by administration of dantrolene sodium. Her serum level of antibodies to glutamic acid decarboxylase (GAD) was markedly elevated and continuous muscular activities were observed on resting surface EMG. These features were compatible with those in stiff-person syndrome (SPS). She was found to have thymoma on CT scan. Immediately after thymomectomy, which was histologically diagnosed as a benign hyperplasia, she developed head retraction reflex-like movements evoked by sensory stimulation to the face, which were followed by severe bulbar symptoms with dysphagia and respiratory arrest. Postoperative myasthenia gravis was excluded clinically. While somatosensory evoked EMG on splenius muscle initially showed biphasic responses with latency of 15 msec and 55 msec, respectively after oral angle non-painful electric stimulation, the late potential phase disappeared after the patient recovered from bulbar symptoms. This suggests that head retraction reflex-like movements of this patient reflected the attenuation of inhibitory potentials from the brainstem.