A family is presented with hyperexplexia, a rare autosomal dominant neurological disorder. Affected individuals manifest flexor hypertonia and hypokinesia during infancy. Later and throughout life, the condition is characterized by exaggerated involuntary myoclonic startle reactions, which on occasion result in falling. There are also marked nocturnal myoclonic jerks. Many family members have had congenital hip dislocations and inguinal hernias. Pre- and postnatal hypertonia is proposed as the cause for these problems. The nature and location of central nervous system dysfunction in hyperexplexia was investigated using electroencephalographic and brainstem-evoked response techniques. A dysfunction of cortical inhibition of the brainstem-mediated startle response is discussed as a possible pathogenic mechanism. Accurate diagnosis of this disorder is important in order to provide appropriate counseling and to initiate effective treatment.