PURPOSE: To characterize the normal bacterial flora and evaluate the presence of periodontopathogenic bacteria around dental implants and to correlate them with the periodontal flora or, in completely edentulous patients, the alveolar gingival flora. MATERIALS AND METHODS: Clinical and radiographic parameters were recorded to exclude peri-implantitis in 34 partially edentulous and 19 completely edentulous patients. Partially edentulous patients were subdivided into two subgroups based on the depth of the periodontal pocket: ≤ 4 mm (n = 19) and > 4 mm (n = 15). Microbial samples were collected from peri-implant sulci, the deepest periodontal sulci, and, for completely edentulous patients, from the alveolar gingiva. Predominant aerobic bacteria were determined by microbiologic culturing, and multiplex polymerase chain reaction was used to detect five periodontopathogenic bacteria: Porphyromonas gingivalis, Tannerella forsythensis, Treponema denticola, Prevotella intermedia, and Actinobacillus actinomycetemcomitans. RESULTS: In all the examined patients, oral streptococci were the most frequent aerobic peri-implant bacteria. The frequency of four periodontopathogenic bacteria in tooth sulci (A actino?mycetemcomitans, P gingivalis, T forsythensis, T denticola) was significantly higher around natural teeth with deeper periodontal pockets, but there was no significant difference in the frequency of the same bacteria in peri-implant sulci in the two partially edentulous subgroups. In contrast, there were no such bacteria in the peri-implant sulci or the alveolar gingiva of completely edentulous patients. CONCLUSIONS: In healthy peri-implant sulci, oral streptococci constitute the predominant bacterial flora. In partially edentulous patients four periodontopathogenic bacteria were detected around implants, and none of these bacteria were found around implants in completely edentulous patients.