Evidence on the role of plaque control in periodontitis is required for several reasons. First, there are increasing pressures to make medical and dental decisions evidence-based. Behavior changes should not be recommended unless evidence exists to support their effectiveness. Personal oral hygiene instruction can create frustrations on the part of both the patient and the periodontist. From a patient perspective, the time burden involved in achieving perfect personal oral hygiene can be daunting. For a periodontist, it can be disappointing to have patients where the goal of improved personal oral hygiene appears unachievable. Such mutual disappointments with the inability of achieving improved personal oral hygiene may lead to discontinuation of effective professional care [7, 12]. Second, poor personal oral hygiene may be used by some health care systems to prevent access to specialist periodontal care and may lead to '2nd class citizen status' in some periodontal practices. If personal plaque control measures are unrelated to chronic periodontitis, such discrimination appears inappropriate. Third, a central hypothesis in periodontal research and care delivery has been that plaque causes periodontitis  and that personal plaque control is essential in controlling chronic periodontitis. Given the high pedestal upon which plaque and personal oral hygiene has been placed in the periodontal hierarchy, evidence is needed to support this leading role. Copyright © Blackwell Munksgaard 2005.