Pertussis has traditionally been considered a disease of young children. In Australia, combined diphtheria–tetanus–acellular pertussis (DTPa) vaccine is currently included in the childhood immunisation schedule at ages 2, 4, 6, and 18 months and, since 1994, before school entry at 4–5 years of age. Prior to 1999, a formulation that included whole cell pertussis vaccine was used. In NSW, periodic epidemics of pertussis have occurred every 3–4 years, most recently in 1993–1994, 1997–1998 and now in 2000–2002. A seasonal pattern is clearly evident with peak notification rates occurring in spring every year.