This study investigated the immediate effects, and the effects during 12 months, of a single episode of root debridement in 1248 sites in 9 periodontitis patients. Single recordings for probing depths and probing attachment levels were made at baseline, and at 3, 6, 9 and 12 months. In addition, triplicate recordings of attachment levels were made for all sites by 3 independent examiners immediately prior to debridement, immediately post debridement, and at 3 and 12 months. It was found that a mean loss of probing attachment of 0.5 to 0.6 mm occurred as a result of instrumentation, irrespective of initial probing depth. Individual sites were identified as having lost probing attachment using a site-specific standard deviation for measurement variability and a greater than or equal to 1.0 mm change. 5% of all sites lost probing attachment from pre-instrumentation to 12 months. Approximately half of these had probing attachment loss inflicted during instrumentation. 23 sites (2% of all sites) were identified as having lost probing attachment from the post-instrumentation time point to 12 months. The majority of these sites seemed to undergo this probing attachment loss as a result of a remodelling process during the healing phase. Over the observation period used in this study, the majority of the attachment loss identified seems to be either directly attributable to instrumentation or to a remodelling process as a result of the therapy rather than to progressive periodontitis.