A review of the literature shows that, currently, no reliable clinical parameter exists either to indicate existing disease activity or to predict future periodontal deterioration. The common clinical parameters used in practice merely assess gingival status or indicate that tissue destruction has occurred earlier (Fig 9). Although many clinicians mistakenly use them as indicators of ongoing destruction, these clinical findings have more value as educational and motivational tools. More sophisticated parameters (that is, antibody tests, and subtraction radiography) are being developed, and their accuracy in identifying disease activity is being tested. These parameters differ in their degrees of accuracy and reliability and, currently, are best suited for research purposes. Quantitative plaque assessments, once shown to have a correlation with incidence of periodontal disease in study populations, have been proved recently to be inaccurate in assessing periodontal disease activity when evaluating specific individuals and sites. Qualitative assessments are more reliable in determining areas that have suffered from specific periodontal disease processes, but their accuracy in identifying ongoing deterioration is questionable. Although it generally is agreed that bacterial plaque is the cause of periodontitis, no plaque assessment to date has been reliable consistently as an indicator of existing or impending periodontal disease activity. Plaque indexes can be used to educate and motivate patients in the removal of this most important etiologic agent. Clinical gingival assessments do not indicate existing or impending periodontal disease activity with loss of attachment and, therefore, are of little value in diagnosing periodontitis. Because gingival inflammation is an undesirable deviation from health, gingival indexes are valuable as a signal to the clinician to manage these patients more closely.(ABSTRACT TRUNCATED AT 250 WORDS)