The objective of this study was to examine inflammatory tissue in deep peri-implant bone pockets (> 5 mm) for anaerobic bacteria colonization. The peri-implant inflammatory tissue of bone defects from 12 edentulous patients with 18 unsuccessful implants (IMZ type) was removed after surgical opening of the defects. After grinding the tissue with glass beads in nutrient solution, an aliquot of the suspension was plated and incubated on appropriate culture media. The quantitative and qualitative distribution of bacteria as a function of the tissue dry weight was determined (cell count/mg dry weight). The mean total cell count was 67 x 10(3) cells/mg dry weight. The following bacteria dominated: species of the family Bacteroidaceae (Prevotella intermedia, Prevotella buccae, Prevotella oralis, Prevotella melaninogenica, Prevotella denticola); Actinobacillus actinomycetemcomitans; Fusobacterium nucleatum; Capnocytophaga spp; and Eikenella corrodens. Bacteroidaceae and Actinobacillus actinomycetemcomitans were found particularly frequently. The increased colonization of these bacteria in deep peri-implant bone pockets is consistent with the currently held view of advanced periodontal lesions, whereby certain pathogens grow at a disproportionate rate in comparison with the total bacteria count under specific circumstances.