Recent evidence extends the health benefits of breastfeeding to include reduction of maternal body mass index (BMI) and childhood obesity. Since most women decide if they will breastfeed prior to pregnancy, it is important to understand, given the high population prevalence of obesity, if maternal underweight, overweight or obese status is associated with breastfeeding initiation. Population-based study. Florida resident birth certificate records. All live singleton births (2004-2009), excluding observations that lacked the primary outcomes of maternal pre-pregnancy BMI and breastfeeding initiation (final sample of 1,161,949 unique observations). Odds of initiating breastfeeding, adjusted by maternal and infant factors, stratified by pre-pregnancy BMI, categorized as underweight, normal, overweight and obese. Adjusting for the known maternal factors associated with breastfeeding initiation, underweight and obese women were significantly less likely to initiate breastfeeding than women with normal BMI, (adjusted odds ratio 0.87, 95 % confidence interval 0.85-0.89 for underweight women; 0.84, 95 % CI 0.83-0.85 for obese women). The magnitude of these findings did not significantly vary by race or ethnicity. Medicaid status and adherence to the Institute of Medicine's 2009 pregnancy weight gain recommendations had only minor influences on breastfeeding initiation. Among adolescents, only underweight status predicted breastfeeding initiation; obesity did not. Underweight and obese women have significantly lower rates of breastfeeding initiation compared to women with normal pre-pregnancy BMI. Future studies need to address the health care, social, and physical barriers that interfere with breastfeeding initiation, especially in underweight and obese women, regardless of race, ethnicity or income.