The objective of this study was to identify biomedical and hospital-related factors associated with the delayed onset of lactation (>72 h postpartum) in a population of Australian women. Subjects were 453 women participating in the second Perth Infant Feeding Study. Information on mothers' perception of the timing of the onset of lactation and associated explanatory factors was collected in a questionnaire completed by women prior to or shortly after discharge from hospital. Multivariate logistic regression was used to identify those factors independently associated with delayed lactation. Risk factors for delayed lactation were being primiparous (adjusted OR 3.16, 95% CI 1.58-6.33) and having delivered by caesarean section (adjusted OR 2.40, 95% CI 1.28-4.51). We failed to find a negative association with maternal body mass index reported in previous studies. While a greater proportion of women who experienced delayed lactation were overweight or obese compared with those who did not experience delayed lactation (40.8% vs. 32.1%), this difference was not statistically significant.