Three major components have been repeatedly implicated for the origin(s) of sudden infant death syndrome (SIDS): system, minor sickness and surroundings. All these factors also frame infant temperament, and therefore it seems logical to suppose that the babies who either succumb to or are at risk of SIDS may present with certain behavioral features. The infants who have died of SIDS moved less when lying awake in the crib, during feeding and during nail cutting; less often turned their head away and looked for their mother when held by a new person; were more frequently reported to show an extreme response (either almost never protested or almost always objected) when approached by someone other than main caregiver; and exhibited extreme behavioral patterns when the infant's vocalizing on waking up was studied. The infants who died of SIDS had lower activity scores. An unfavorable microenvironment can increase the risk of SIDS, and the babies facing less developmental stimulation had a more negative mood and were less distractible; less organized infants presented with a more negative mood, less distractible behavior, lower rhythmicity, lower persistence and lower adaptability. The infants born to smoking mothers who are at risk of SIDS had more intensive reactions. The low-birth-weight infants who are at high risk of SIDS were more withdrawing and less adaptable. Infant--parent(s) bed sharing may increase the risk of SIDS, and solitary sleeping infants had a more positive mood and were more persistent. Use of a pacifier may be protective against SIDS, and pacifier users presented with higher rhythmicity. Prone sleep is known to increase the risk of SIDS, and the lowest persistence was a feature of those babies who were usually put to sleep supine and found prone. Infants who snored and/or had noisy breathing in sleep were characterized by more negative mood. Infants with signs of repetitive regurgitation were less distractible.