During paroxysms of colic, infants are hypertonic or neurolabile, and appear to be in acute, abdominal pain. The infant lets out a high-pitched cry which soon reaches a screaming level, and which is coupled with facial grimacing. The infant is difficult to console, and may resist attempts to sooth it. Between spells, these infants cry normally and are not hypertonic. Recent research suggests that infants with true colic represent a subgroup of infants with excessive crying. Findings indicate that colic and excessive crying can be differentiated by physical and behavioral criteria. Mothers of infants with colic perceive their infant's cry as being different from that of other infants, and rate their infants as having a difficult temperament. From a pediatric point of view, colic appears to be an imbalance of the autonomic nervous system, particularly of the dynamic interplay between sympathetic and parasympathetic systems. From a developmental point of view, this autonomic imbalance could be related to the broad spectrum of changes in neurological and behavioral function that occur around the second month: the so-called biobehavioral shift from basic physiological regulation to the beginnings of social regulation. What is triggered by crying as a biologically based condition may develop into crying as mediated by social-emotional factors in the parents. (RH)