Purpose: To evaluate, by means of the polymerase chain reaction (PCR), the presence of Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, and Prevotella intermedia in the mandibular arch of completely edentulous subjects before implant placement and 4 and 6 months after the placement of mandibular implant-supported fixed prostheses. materials and methods: Fifteen patients had bacterial plaque collected with sterile paper points before implant placement (ie, when they were completely edentulous) and at 3 sites on the peri-implant sulci displaying the largest probing depths after placement of 5 implants. Results: For the edentulous arch, A actinomycetemcomitans was detected in 13.3% of subjects, P intermedia was detected in 46.7% of subjects, and there was no detection of P gingivalis. After 4 and 6 months of implant placement, A actinomycetemcomitans was detected in 60% and 73.3% respectively; P intermedia in 46.7% and 53.3% respectively; and P gingivalis in 46.7% and 53.3%, respectively. Discussion: Future diagnosis should not be restricted to distinguishing individuals at risk of peri-implant disease. Such individuals should be identified by the employment of microbiologic methods and knowledge of the multifactorial nature of the host response to the action of microorganisms. Conclusions: The longer the implants were in the oral cavity, the higher the occurrence of A actinomycetemcomitans, P gingivalis, and P intermedia in the peri-implant sulci of completely edentulous patients rehabilitated with mandibular implant-supported fixed prostheses was, without any clinical or radiographic evidence indicating peri-implant disease in the studied period. (Before-and-After Study)