OBJECTIVE: The objective of this study was to identify the factors associated with exclusive breastfeeding in children aged less than six months from the Brazilian Legal Amazon and Northeast regions. METHODS: The study used data from a survey that assessed prenatal and infant (<1 year) care in 2010. Sociodemographic, prenatal, delivery, and puerperium care factors with p<0.05 in multivariate analysis were associated with exclusive breastfeeding. RESULTS: For both regions, the prevalence of exclusive breastfeeding decreased with age, which was the main variable associated with early weaning. In the Legal Amazon, exclusive breastfeeding prevailed among: mothers aged 35 years or more; mothers living in state capitals; and mothers who breastfed on the first hour of life. In the Northeast, the probability of exclusive breastfeeding was greater for mothers aged 35 years or more. CONCLUSION: The factors associated with exclusive breastfeeding were child's and mother's age in both regions; and residence location and breastfeeding in the first hour of life in the Legal Amazon, suggesting the need of differentiated strategies for the promotion of exclusive breastfeeding.