Voluntary movements, such as the self-paced finger extension task used in the present experiments, are preceded by a slowly rising negative electroencephalographic potential [the movement-related cortical potential (MRCP)]. The early NS1 component of the potential was no different in patients with primary dystonia affecting the arm (n = 6) compared with matched controls. In contrast, the peak amplitude of the MRCP was smaller in the patients, despite the fact that the movements made by the two groups were very similar; it was of equal size over both left and right hemispheres, rather than being larger on the side contralateral to the movement. These results are similar to those observed by others in patients with symptomatic dystonia secondary to lesions of the basal ganglia or their output pathways and may reflect abnormal basal ganglia input to motor areas of cortex before the onset of a self-paced movement.