Data from CPITN studies indicate that severe periodontitis affects approximately 10 per cent of most populations. These data have remained static for a number of years. Of interest, however, is that despite the dramatic increase in the use of oral hygiene aids, efforts by the dental profession in oral hygiene instruction, and the associated general improvement in oral hygiene levels in the community, the incidence of severe chronic inflammatory periodontal disease has remained largely unaffected. The effects of changing oral hygiene may be reflected in slight shifts in the mild and moderate classifications of periodontal disease but the prevalence of advanced disease in presumably susceptible subjects has remained relatively unchanged. The ramifications of relatively non-specific plaque control measures in the management of advanced disease in susceptible subjects are still unclear and it may not be until the adoption of a more specific approach to the control of specific pathogens which inhabit the subgingival biofilm that major changes in the general incidence of the severe inflammatory periodontal diseases will be seen.