This study determined the microbiota of the mucosa- and implant facing parts of expanded polytetrafluoroethylene augmentation material, and the influence of major periodontopathogens on the healing process associated with guided bone regeneration around dental implants. Seventeen patients with nine dehiscence and eight extraction defects were studied. Prior to surgery and at membrane removal, microbial morphotypes, total viable counts, and the occurrence of selected microbial species were examined by phase-contrast microscopy, nonselective and selective cultures, and DNA probes. Nine sites with submerged barrier membranes throughout the 9-month study were free of cultivable microorganisms and experienced significantly more osseous healing than eight sites with prematurely exposed membrane. Patients with few or no deep periodontal pockets demonstrated significantly fewer residual osseous defects than patients showing several pockets of increased depths. In addition, patients with prematurely exposed membranes revealed several deep periodontal pockets. Three patients with less than 1 mm of osseous gain yielded either Porphyromonas gingivalis or Actinobacillus actinomycetemcomitans. Peptostreptococcus micros occurred in high proportions in seven of the eight patients with premature membrane exposure and inadequate osseous healing. These findings associate putative periodontal pathogens with unsuccessful guided bone regeneration. The control of periodontal pathogens in the oral cavity prior to placement of barrier membranes around implants might increase the prognosis of osseous regeneration. Keywords: guided tissue regeneration, implants, pathogenic bacteria, Peptostreptococcus micros, Porphyromonas gingivalis