The purpose of this clinical investigation was to compare the new resorbable collagen membrane, Bio-Gide, to the conventional expanded polytetrafluoroethylene material (Gore-Tex) for guided bone regeneration in situations involving exposed implant surfaces. Over a 2-year period, 25 split-mouth patients were treated randomly: one defect site was treated with Bio-Gide and the other defect site with Gore-Tex; all 84 defects were filled with Bio-Oss and covered with the respective membrane. The defect types, their dimensions, and their morphology were measured in detail initially and at re-entry to allow for calculation of the exposed implant surface. Changes in defect surface for both types of membranes were statistically significant (P .0001); however, no statistical significance (P > .94) could be detected between the two membranes. The mean average percentage of bone fill was 92% for Bio-Gide and 78% for Gore-Tex sites. In the latter group, 44% wound dehiscences and/or premature membrane removal occurred. The resorbable membrane, Bio-Gide, in combination with a bone graft, can be a useful alternative to the well-established expanded polytetrafluoroethylene membranes. Schlagwörter: biodegradable membranes, Bio-Oss, defect morphology, guided bone regeneration, implants, membrane exposures, nonresorbable membranes, timing of implant placement