The duration of breast-feeding and the growth of fully breast-fed infants during the first year of life were analyzed according to sex, birth weight, characteristics of the mother, and breast-feeding pattern. All infants (n = 1217) were healthy and fully breast-fed at 30 d postpartum and 63% and 24% were still fully breast-fed at 6 and 12 mo, respectively. The median ages at introduction of nondairy food and of milk supplements were 6.0 and 7.4 mo, respectively. The probability of remaining fully breast-fed for 12 mo was significantly higher in infants with higher birth weight and higher maternal weight, and in infants who breast-fed seven times a day or more in the first 6 mo. The comparison of the monthly weight and length of fully breast-fed infants with the World Health Organization/National Center for Health Statistics reference data showed the adequacy of breastfeeding to support infant growth. When mothers and infants are healthy, breast milk is sufficient to support adequate infant growth and health during the first months of life. The main variables that affect the duration of breast-feeding, nutrition and suckling frequency, are susceptible to interventions by health services. The results reinforce the need to care for maternal nutrition during pregnancy and to provide the support that women need to sustain a high suckling frequency.