Objectives: To describe the epidemiology of pertussis, and to identify changes in the source of pertussis in infants 6 months of age and under, during the 2008-2012 epidemic in south metropolitan Perth., Design and Setting: Analysis of all pertussis cases notified to the South Metropolitan Population Health Unit and recorded on the Western Australian Notifiable Infectious Disease Database over the study period. Information on the source of pertussis was obtained from enhanced surveillance data., Results: Notification rates were highest in the 5-9 years age group, followed by the 0-4 years and 10-14 years age groups. There was a significant increase in the proportion of known sources who were siblings from the early epidemic period of 2008-2010, compared with the peak epidemic period of 2011-2012 (14.3% versus 51.4%, p = 0.002). The majority of sibling sources were fully vaccinated children aged 2 and 3 years., Conclusions: The incidence of pertussis was highest in children aged 12 years and under in this epidemic. At its peak, siblings were the most important sources of pertussis in infants 6 months and younger, particularly fully vaccinated children aged 2 and 3 years. Waning immunity before the booster at 4 years may leave this age group susceptible to infection. Even if cocooning programs could achieve full vaccination coverage of parents and ensure all siblings were fully vaccinated according to national schedules, waning immunity in siblings could provide a means for ongoing transmission to infants. Recent evidence suggests that maternal antenatal vaccination would significantly reduce the risk of pertussis in infants 3 months of age and under.