The literature on familial periodic paralysis has revealed no cases of intercurrent psychosis. The patients have been described by several authors as being relatively free of emotional difficulties, and this freedom has been attributed to the absence of cerebral pathology. A case is presented in which there was an acute psychotic episode during the course of familial periodic paralysis. The patient also had a sociopathic personality disturbance. Psychological test findings are presented, as well as the course of the psychosis. The relation of the patient's psychiatric symptoms and psychological test results to both his organic illness and his personal history is discussed.Abstract Teaser