Hyperexplexia is a disease of neonatal onset characterized by an exaggerated startle reflex. Early diagnosis is important to rule out epilepsy. Clinical findings are mainly hypertony and generalized startle reflex which is exaggerated by tiredness and some exogenous stimuli. Electroencephalogram is normal. The expression of the disease is variable including minor forms that may be unnoticed and major forms with arthrogryposis-like symptoms, orthopedic complications, false passages, apnea and even sudden infant death (SID). The evolution is generally benign and symptoms disappear within 2 or 3 years. A neuromotor retardation is often present, without intellectual deficit. In severe forms, the risk of SID requires a multidisciplinary follow-up including monitoring and treatment with clonazepam. A low GABA level in cerebrospinal fluid has been reported. Present etiological hypotheses include neuromediator and/or receptor dysfunction.