Four generations of a family are described in which 7 of 8 affected members suffered from prolonged dystonic seizures; the eighth member's attacks were those of paroxysmal choreoathetosis. The attacks lasted up to 4 hours, were precipitated by alcohol, emotion, or fatigue, and responded poorly to phenytoin and barbiturates but were controlled by clonazepam. Autopsy on an affected child who died a "crib death" at the age of 2 years disclosed no major abnormality of the brain. This family appears to have the same condition as that described by Mount and Reback in 1940, Forssman in 1961, and Richards and Barnett in 1968, which the last authors termed paroxysmal dystonic choreoathetosis to distinguish it from the more common movement-induced (kinesigenic) form of the disorder. Analysis of reports of 100 cases of paroxysmal kinesigenic choreoathetosis shows that the attacks last less than 5 minutes, are precipitated by sudden movement or startle, and usually respond well to phenytoin or barbiturates. A clinical classification is presented in which the kinesigenic form is divided into a familial group (72% of cases) and a sporadic group (28% of cases) and contrasted with paroxysmal dystonic choreoathetosis. One family with an intermediate form, in which dystonic choreoathetosis was provoked by continued exertion and lasted for up to 30 minutes, is also reported.