Hyperekplexia is a hereditary neurologic disorder manifested by an exaggerated startle response, generalized muscular rigidity, and prominent nocturnal myoclonus. The distinctive features of this syndrome constitute an unusual clinical entity that is easily mistaken for other disorders. The study of a family provided additional data on various aspects of this condition. The proband was 3 months old when he was referred for persistent generalized stiffness since birth; this stiffness was associated with an exaggerated startle response and intermittent apnea. Similar symptoms in infancy and prominent nocturnal myoclonus with an excessive startle response in adulthood were described in other family members for five generations. Various features of the disorder indicate a relationship to the syndrome of the "jumping Frenchmen of Maine," latah, miryachit, and other unusual startle reactions.