Fifteen patients with hyperekplexia were identified in 3 families; diagnostic clinical characteristics were defined which allowed for early recognition and treatment. During the first 24 hours of life, spontaneous apnea and sluggish feeding effort were observed. After the first 24 hours, surviving infants exhibited the hyperekplexic startle response to nose tapping. This startle response is characterized by sudden muscular rigidity, feeding-induced oropharyngeal incoordination, and poor air exchange often with apnea, persisting with repetitive nose tapping. Untreated infants experienced recurring apnea until 1 year of age. Three of 15 patients died unexpectedly during the neonatal period. Patients treated with clonazepam (0.1-0.2 mg/kg/day) had no serious apneic episodes and startle reflexes were diminished. The pathophysiologic mechanism for hyperekplexia remains obscure. Electroencephalographic studies were consistently normal. The response to and tolerance of benzodiazepines are striking in newborns and infants and suggest an aberrant central nervous system reflex as the etiology; therefore, hyperekplexia should be considered in the evaluation of neonates and infants with apnea, aspiration pneumonia, episodic muscular rigidity, hyperexcitability, and near-miss sudden infant death syndrome. The need for immediate monitoring of at-risk infants, observation for signs of hyperekplexia, and initiation of clonazepam in these patients are emphasized. Hyperekplexic startle response to nose tapping should be included in the routine examination of all newborns.