BACKGROUND: In-hospital formula supplementation of breastfed newborns is commonplace despite its negative association with breastfeeding duration. Although several studies have described the use of formula supplementation, few have explored the factors that may be associated with its use. The aim of this study was to explore factors associated with in-hospital formula supplementation using data from a large Australian population-based survey. METHODS: All women who gave birth in September and October 2007 in two Australian states were mailed questionnaires 6 months after the birth. Women were asked how they fed their baby while in hospital after the birth. Multivariable logistic regression was used to explore specified a priori factors associated with in-hospital formula supplementation. RESULTS: Of 4,085 women who initiated breastfeeding, 23 percent reported their babies receiving formula supplementation. Breastfed babies had greater odds of receiving formula supplementation if their mother was primiparous (adj. OR=2.16; 95% CI: 1.76-2.66); born overseas and of non-English-speaking background (adj. OR=2.03; 95% CI: 1.56-2.64); had a body mass index more than 30 (adj. OR=2.27; 95% CI: 1.76-2.95); had an emergency cesarean section (adj. OR=1.72; 95% CI: 1.3-2.28); or the baby was admitted to a special care nursery (adj. OR=2.72; 95% CI: 2.19-3.4); had a birthweight less than 2,500 g (adj. OR=2.02; 95% CI: 1.3-3.15) or was born in a hospital not accredited with Baby-Friendly Hospital Initiative (BFHI) (adj. OR=1.53; 95% CI: 1.2-1.94). CONCLUSIONS: The number of factors associated with in-hospital formula supplementation suggests that this practice is complex. Some results, however, point to an opportunity for intervention, with the BFHI appearing to be an effective strategy for supporting exclusive breastfeeding.