The purpose of the present study was to investigate the clinical differences between individuals highly susceptible and individuals insusceptible to periodontal breakdown. The susceptible group consisted of 7 patients with a diagnosis of juvenile periodontitis. The insusceptible group consisted of 7 individuals selected on the basis of age (52 years or older), presence of at least 18 teeth, no evidence of extractions due to periodontal breakdown, no loss of attachment, shallow pockets, gross amounts of plaque and no history of interdental cleaning. Results showed that the susceptible group had more bleeding, less plaque and deeper pockets than the insusceptible group. Since the 2 groups seem to behave differently with regard to bleeding upon probing and amount of plaque, bleeding/plaque ratios were calculated. Testing showed a highly significant difference between the 2 groups. The same results were obtained if only sites with shallow pockets were included in the analysis. On the basis of these results, it is hypothesized that a high value of the ratio between bleeding and plaque may act as a prognostic indicator for periodontal breakdown.