The objective of this study was to identify the impact of pacifier use on the duration of breastfeeding amongst Australian women. A cohort of 556 mothers who delivered in Perth, Western Australia was recruited to study their infant feeding practices. The mothers were interviewed in hospital and again at 2, 6, 10, 14, 18 and 24 weeks postpartum, or until they ceased to breastfeed. At two weeks 62% of breastfed babies were using a pacifier, increasing to a peak of 78% at six weeks. Infants who were using a pacifier had slightly fewer feeds each day at every age period (for example 6.9 versus 7.4 feeds at six weeks of age), but there was no difference in the number of night feeds. A recent study suggested that the mothers resorted to the use of pacifiers when they were having problems breastfeeding, and any impact of pacifiers on breastfeeding duration was due to confounding factors. However in this longitudinal study, after adjusting for the presence of breastfeeding problems, the use of a pacifier at two weeks was associated with reduced likelihood of breastfeeding to six months (odds ratio 0.40; 95%CI 0.25-0.63). Based on the results of this study we concluded that the use of a pacifier at two weeks of age reduced the likely duration of breastfeeding to six months. A possible mechanism of action was the reduced number of daily feeds in breastfed infants that would reduce breast stimulation. If mothers choose to use a pacifier they should introduce it later and use it infrequently.