The study aimed to assess factors associated with exclusive breastfeeding in low birthweight infants under six months of age in 64 Brazilian municipalities. This cross-sectional study included the 2,745 low birth weight infants under six months of age in the Survey on Prevalence of Breastfeeding (2008) in Brazilian municipalities. The outcome was exclusive breastfeeding. Individual determinants included socioeconomic characteristics (infants' age and sex), maternal characteristics (age bracket, work status, and parity), and characteristics of health services (birth in a Child-Friendly Hospital; place of outpatient follow-up; type of delivery). Two contextual determinants were also included: "number of Human Milk Banks per municipality per 10,000 live births" and "municipal human development index". The individualized effect of the study factor on the outcome was assessed by multilevel Poisson regression with level of significance under 5%. Exclusive Breastfeeding was more prevalent in low birth weight infants whose mothers were 20-35 years of age, who were not working away from home or were on maternity leave, who had given birth in a Child-Friendly Hospital, and who lived in municipalities with more human milk banks per 10,000 live births. The result of the positive and independent effect of strategies that were already part of the National Breastfeeding Policy (birth in a Child-Friendly Hospital, living in a municipality with more human milk banks, and adopting laws that guarantee maternity leave) on exclusive breastfeeding in low birth weight infants points to the need to expand the coverage of these measures in order to meet the breastfeeding targets proposed by the World Health Organization.