DiPietro, Larson, and Porges (1987) found behavioral and physiological differences between breast-fed and bottle-fed newborns. It was suggested that breast-feeding is associated with more optimal physiological organization and with increased irritable reactivity early in the neonatal period. The present study investigated whether breast-fed neonates' more optimal physiological organization leads to more optimal development later in infancy. A total of 29 breast-fed and 15 bottle-fed infants who had participated in a newborn study were seen again at 15 months of age. During the assessment of newborns, 10 minutes of heart pattern and respiration data were collected and the Neonatal Behavioral Assessment Scale was administered. At 15 months, the Bayley Scales of Infant Development and the Infant Characteristics Questionnaire were administered. Findings revealed that breast-fed infants scored significantly higher than bottle-fed infants on the Bayley Mental Development Index (MDI) at 15 months of age and tended to be rated as more fussy-difficult-demanding by their parents at 15 months. The decision to breast-feed was related to outcome at both the newborn period and at 15 months of age. Level of parent education and duration of breast-feeding were not related to outcome. It is concluded that, insofar as the predictive influence of feeding style is largely accounted for by neonatal vagal tone, that variable may mediate the observed developmental differences. (RH)