An osteopromotive technique has been developed that allows improved bone regeneration as well as bone neogenesis using porous, inert, nondegradable membranes made of expanded polytetrafluoroethylene (e-PTFE). For certain applications, however, it would be advantageous to use bioabsorbable membranes (BAMs), thus avoiding a surgical reentry for membrane removal. In this randomized comparison study the osteopromotive potential of BAMs was investigated in standardized "critical size defects" (5 mm in diameter) in the rat mandible. Three membrane types were tested and comparisons were made with e-PTFE membrane. The BAMs consisted of polylactic/polyglycolic acid copolymers designed to give the membranes different absorption times when implanted in the tissue. Histologic analysis after healing periods of 1 to 12 weeks demonstrated the BAMs to be well tolerated by the tissue, causing just a mild inflammatory reaction along the membrane surfaces as long as the material remained in the tissue. The BAMs were found to be as efficient as e-PTFE membranes in that the bone repair was not significantly different with any of the four membrane types. However, healing in conjunction with one type of BAM seemed to occur somewhat more rapidly. Some cartilage was present at the early healing stages in the defects treated with BAMs, but disappeared at later stages. The results of this study show that BAMs are a valid alternative to e-PTFE membranes to improve bone regeneration, but indicate that further technical development of the membrane material is necessary.