Paroxysmal kinesigenic dyskinesia (PKD) is characterized by brief episodes of choreic/dystonic movements precipitated by sudden movement. The condition responds to antiepileptic medication, particularly carbemazepine. Autosomal dominant inheritance is often seen, and a locus in the pericentromeric region of chromosome 16 has been identified in some families. Little is known of the pathophysiology of PKD, although an ion channel abnormality is thought likely. We assessed a number of electrophysiological parameters in 11 patients with idiopathic PKD, a proportion of them on and off treatment. We identified reduced short intracortical inhibition (SICI), reduced early phase of transcallosal inhibition, and a reduced first phase of spinal reciprocal inhibition (RI) in subjects with PKD. The cortical silent period, the startle response and the second and third phases of RI were normal. Treatment with carbamazepine normalized the abnormalities in transcallosal inhibition, but had no effect on other parameters. Patients with PKD show a discrete set of abnormalities in cortical and spinal inhibitory circuits that differ from those seen in primary dystonia and epilepsy, and which may provide clues to the underlying pathophysiology of the disorder.