Background: Paroxysmal kinesigenic dyskinesia (PKD) is a rare disorder charac terized by short episodes of involuntary movement attacks triggered by sudden vo luntary movements. Although a genetic basis is suspected in idiopathic cases, th e gene has not been discovered. Establishing strict diagnostic criteria will hel p genetic studies. Methods: The authors reviewed the clinical features of 121 af fected individuals, who were referred for genetic study with a presumptive diagn osis of idiopathic PKD. Results: The majority (79% ) of affected subjects had a distinctive homogeneous phenotype. The authors propose the following diagnostic criteria for idiopathic PKD based on this phenotype: identified trigger for the attacks (sudden movements), short duration of attacks ( < 1 minute), lack of lo ss of consciousness or pain during attacks, antiepileptic drug responsiveness, e xclusion of other organic diseases, and age at onset between 1 and 20 years if t here is no family history (age at onset may be applied less stringently in those with family history). In comparing familial and sporadic cases, sporadic cases were more frequently male, and infantile convulsions were more common in the fam ilial kindreds. Females had a higher remission rate than males. An infantile-o nset group with a different set of characteristics was identified. A clear kines igenic trigger was not elicited in all cases, antiepileptic response was not uni versal, and some infants had attacks while asleep. Conclusions: The diagnosis of idiopathic paroxysmal kinesigenic dyskinesia (PKD) can be made based on histori cal features. The correct diagnosis has implications for treatment and prognosis , and the diagnostic scheme may allow better focus in the search for the PKD gen e(s). Background: Paroxysmal kinesigenic dyskinesia (PKD) is a rare disorder characterized by short episodes of involuntary movement attacks triggered by sudden vo luntary movements. Although a genetic basis is suspected in idiopathic cases, th e gene has not been discovered. Establishing strict diagnostic criteria Methods: The authors reviewed the clinical features of 121 af fected individuals, who were referred for genetic studies with a presumptive diagnosis of idiopathic PKD. Results: The majority (79%) of affected subjects had a distinctive homogeneous phenotype. The authors propose the following diagnostic criteria for idiopathic PKD based on this phenotype: identified trigger for the attacks (sudden movements), short duration of attacks (<1 minute), lack of lo ss of consciousness or pain during attacks, antiepileptic drug responsiveness, e xclusion of other organic diseases, and age at onset between 1 and 20 years if t here is no family history (age at o In comparing familial and sporadic cases, sporadic cases were more frequently male, and infantile convulsions were more common in the fam ilial kindreds. Females had a higher remission rate than males. An infantile A clear kines igenic trigger was not elicited in all cases, antiepileptic response was not uni versal, and some infants had attacks while asleep. Conclusions: The diagnosis of idiopathic paroxysmal kinesigenic dyskinesia ( PKD) can be made based on histori cal features. The correct diagnosis has implications for treatment and prognosis, and the diagnostic scheme may allow better focus in the search for the PKD gen e (s).