9 adult subjects with severe periodontitis were monitored following oral hygiene instruction and a single episode of crown and root debridement. Baseline recordings for probing attachment level were obtained both immediately pre-instrumentation and immediately post-instrumentation. Sites with gain, no change or loss of probing attachment level at 3 and 12 months compared to both pre- and post-instrumentation were identified. The classification was based upon the use of triplicate recordings at each time point, a site-specific standard deviation for measurement variability, and the requirement of a minimum of 1.0 mm change. The relative frequencies of gain, no change, and loss of probing attachment were then calculated for sites of various residual probing depths at 3 and 12 months. This was performed to evaluate if a given probing depth at re-evaluation, e.g., 7.0-7.5 mm, could be used as an indicator of the need for supplementary treatment following the initial therapy, based upon the observed probing attachment changes compared to baseline. As an example of the results of the present study, 60% of sites with residual probing depths of 7.0-7.5 mm showed probing attachment gain greater than or equal to 1.0 mm compared to the post-instrumentation baseline, and only 2% had undergone probing attachment loss greater than or equal to 1.0 mm. The overall results suggest that a relatively deep residual probing depth at re-evaluation following initial therapy, by itself, provides little evidence of lack of improvement compared to baseline. On this basis, the use of a specific probing depth at 3 or 12 months following treatment as a yardstick for the provision of supplementary treatment may not be justified.