Reflex sympathetic dystrophy (RSD) is a syndrome dominated by sensory, autonomic, and motor features of the extremities. In this study, 10 severely affected RSD patients who progressed to multifocal or generalized tonic dystonia underwent H-reflex evaluation, needle electromyography (EMG), polysomnography, somatosensory evoked potentials, and transcranial magnetic stimulation. H-reflex evaluation revealed an impaired vibratory inhibition of the H-reflex and a higher facilitation peak in the recovery curve between 200 to 350 msec. Needle EMG revealed an impaired reciprocal inhibition, and many patients were unable to alter the amount of muscle activity voluntarily. Evaluations of the stretch reflex showed a markedly decreased threshold and abnormal responses to tonic and phasic changes. Polysomnography performed in five patients revealed no abnormal EMG activity during nonrapid eye movement and rapid eye movement sleep, but EEG arousal phenomena provoked abnormally high and brief bursts of surface EMG activity in all registered muscle groups. Somatosensory evoked potentials and transcranial magnetic stimulation were normal. Taken together, the findings in these patients with tonic dystonia of RSD are in accordance with an impairment of inhibitory interneuronal circuits at the level of the brainstem or spinal cord.