Bleeding on probing (BOP) is a widely used criterion to diagnose gingival inflammation. The purpose of the present retrospective study was to evaluate its prognostic value in identifying sites at risk for periodontal breakdown during the maintenance phase of periodontal therapy. 55 patients who had been treated for advanced periodontitis participated in a recall system for at least 4 years, at regular intervals of 3-5 months. At the start of every appointment, BOP to the bottom of the pocket was registered at 4 sites of every tooth. A random selection of 1054 pockets was made and subdivided into 5 categories according to the incidence of BOP during the last 4 recall appointments. All pockets with a BOP incidence of 4/4 and 3/4 were selected, while only interproximal sites with a BOP incidence of 2/4, 1/4 and 0/4 were chosen. Subsequently, these categories were grouped according to whether or not the attachment level had been maintained from the time prior to the last 4 recall visits. Greater than or equal to 2 mm was defined as loss of clinical attachment. The results indicated that pockets with a probing depth of greater than or equal to 5 mm had a significantly higher incidence of BOP. Patients with 16% or more BOP sites had a higher chance of loosing attachment. Pockets with an incidence of BOP of 4/4 had a 30% chance of loosing attachment. This chance decreased to 14% with BOP of 3/4, 6% with BOP of 2/4, 3% with BOP of 1/4 and 1.5% with BOP of 0/4.(ABSTRACT TRUNCATED AT 250 WORDS)