To identify determinants of the initiation and duration of breastfeeding amongst Australian women. A prospective cohort study of 556 women in Perth, Western Australia and 503 women from the Darling Downs area, Queensland, Australia. Breastfeeding at discharge was most strongly associated with perceived paternal support of breastfeeding with an adjusted odds ratio of 9.13 (95% CI 4.83-17.26), using multivariate logistic regression analysis. Duration of breastfeeding was most strongly associated with the length of time a mother intended to breastfeed with an adjusted relative risk of 4.18 (95% CI 2.81-6.22) for > or = 4 months relative to < 4 months. Interventions which aim to increase the length of time a woman intends to breastfeed, and which highlight the role of the father in successful breastfeeding, are recommended to help achieve recommended targets for breastfeeding initiation and duration.