Although invasive pneumococcal disease (IPD) accounts for only a minority of the infections caused by Streptococcus pneumoniae (such as pneumonia and otitis media), it is associated with the most severe disease and is readily measured, as it is by definition associated with a sterile site isolate. The Metropolitan NSW Pneumococcal Study Group was formed in 1997 to generate data describing the age-specific incidence, serotype distribution and antimicrobial resistance patterns of invasive pneumococcal disease in a large population that is believed to be representative of urban Australia.