The purpose of this study was to investigate the prevalence of the microbiota on the oral mucosal surfaces and in the peri-implant pocket in edentulous subjects with a past history of periodontitis. Twenty edentulous subjects wearing dental implants for at least one year and with a history of periodontitis participated in this study. Clinical parameters were assessed (plaque, redness, swelling, peri-implant probing depth [PIPD], and bleeding on probing) and microbiological samples were taken (oral mucosal surfaces, saliva, plaque, and peri-implant pocket). Based on the clinical results, the group as a whole showed healthy peri-implant tissues. A mean PIPD of 3.6 mm was found. A statistically significant correlation was found between the PIPD and bleeding upon probing, plaque scores, redness, and swelling. Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis were not detected. The subjects harboring Prevotella intermedia showed presence of PIPD of > or = 5 mm. All subjects harbored Peptostreptococcus spp., Fusobacterium spp., and other Prevotella species. Actinomyces odontolyticus, Bacteroides forsythus, Campylobacter rectus, Pseudomonas spp., and enterobacteria were detected less frequently. Three subjects showed signs of overt soft tissue inflammation. They harbored higher proportions of Peptostreptococcus spp. than the other subjects. The results corroborate the suggestion that the primary source of colonization for dental implants in edentulous patients are the oral mucous membranes. The results indicate that the microbiota of subjects wearing dental implants with a past history of periodontitis are composed of bacteria associated with a healthy periodontium or gingivitis. It is suggested that elimination of the subgingival environment by extraction of all natural teeth probably initiates the disappearance of the two periodontal bacteria A. actinomycetemcomitans and P. gingivalis.