In-hospital formula supplementation of breastfed infants negatively impacts breastfeeding duration. Infants from low-income families have some of the lowest exclusive breastfeeding rates in the United States. The objectives of this study were to identify (1) reasons low-income breastfeeding mothers begin in-hospital formula supplementation and (2) risk factors for in-hospital formula supplementation. We surveyed 150 low-income mothers in a Washington, DC, clinic. Sixty percent had initiated breastfeeding, and 78% of these breastfed infants received formula supplementation in the hospital. There was no clear medical need for supplementation for 87% of the breastfed infants receiving supplementation. Infants of mothers who did not attend a prenatal breastfeeding class were almost 5 times more likely to receive in-hospital formula supplementation than those infants whose mothers had attended a class (OR, 4.7; 95% CI, 1.05-21.14). Improved knowledge about breastfeeding among nursing and medical providers is important to minimize unnecessary formula supplementation for breastfed infants.