Sixteen intrabony defects in 12 patients were treated by gingival flap surgery including root surface debridement and placement of an expanded polytetrafluoroethylene (ePTFE) membrane. The membranes were removed after 4 to 6 weeks and examined by scanning electron microscopy (SEM) for bacterial contamination and adherent connective tissue elements. Twelve months postsurgery, the defect sites were reexamined for changes in probing attachment level and probing bone level. Comparison of ultrastructural findings and clinical observations revealed that extent of bacterial contamination of the membrane correlated inversely with clinical assessment of attachment gain. The results indicate that the extent of oral exposure and bacterial contamination of the ePTFE membrane at the time of removal may be an indicator of the long-term success or failure of the regenerative procedure.