A study of the incidence, evolution, and pathophysiology of so-called "breathholding spells" has been carried out prospectively, retrospectively, and physiologically in three groups of patients. The term of infantile syncope is proposed for these attacks initiated by a noxious stimulus and in which both consciousness and posture are lost. Convulsive components may occur if cerebral anoxia is marked. The degree of breathholding varies considerably among attacks, but is prominent in a majority. From the clinical point of view, breathholders could be assigned to a cyanotic group, a pallid group, or an indeterminate group. Such spells occurred in 4.6% of 4,980 individuals, and those affected tended to have a positive family history of such attacks. The onset of such attacks varied from the neonatum to the middle of the fourth year, the commonest ages being between 6 and 18 months. Precipitating factors consisted of mild injuries, frustration, or fright, and the factor of surprise seemed important. From the retrospective point of view the attacks ceased spontaneously at or before school age. Epilepsy and mental deficiency were unrelated to breathholding attacks. Syncopal attacks in adulthood were a frequent sequel of breathholding attacks in infancy. Differentiation into two physiologically diverse groups of cases could be made by monitoring the results of the oculocardiac reflex with simultaneous electrocardiogram and electroencephalogram. Among cyanotic breathholders the response to ocular compression tended to be the usual one of bradycardia or brief asystole. Among the pallid group, cardiac asystole was more prolonged and electroencephalographic abnormality paralleled it. The prognosis for either type was excellent. Differentiation from epilepsy was important, since treatment for epilepsy was usually unrewarding and often pushed to toxic levels. In the pallid group with many attacks the use of atropine has been helpful, but treatment in most instances was unnecessary.