A healthy intelligent 13.5-year-old boy is reported who presented with paroxysmal kinesigenic choreoathetosis. He had had unilateral tonic spasms for 2 months. In a stressful situation, especially after a period of immobility, a sudden voluntary movement provoked a dystonic-choreoathetoic posturing of the left arm and leg. Attacks were brought on, for instance, by standing up quickly from a sitting position. They lasted only 5-10 s, without loss of consciousness and occurred up to ten times daily. Between the attacks there were no abnormalities. Family history was non-contributory. Neurological examination and laboratory findings, including repeated EEGs, were all negative. The child responded very well to phenytoin treatment and has now been completely free of dystonic attacks for a period of 9 months. The importance of early recognition of this syndrome and the differential diagnosis with reflex epilepsy and other forms of dystonic choreoathetosis is discussed.