Background: Guided bone regeneration (GBR) is a widely used procedure for augmenting alveolar ridge width prior to placement of endosseous implants. Various graft materials and barrier membranes (non-resorbable and bioabsorbable) have been used in GBR. The aim of this study was to assess the performance of a new bioabsorbable, synthetic polyglycolic acid/trimethylene carbonate (PGA/TMC) barrier membrane with an increased absorption time in conjunction with a combination of assayed demineralized bone matrix and cortical cancellous chips uniformly dispersed in a thermoplastic biologic carrier. Methods: At 72 potential implant sites in 38 subjects, ridge width at the crest and 4 mm apical to the crest was measured before and 6 months after a GBR procedure using the long-term (LT) PGA/TMC membrane and an allograft in a thermoplastic carrier. Before placement of endosseous implants, 48 biopsy specimens were obtained from the augmentation sites and analyzed histomorphometrically. Results: The GBR procedure increased the mean ridge width at the crest from 2.4 to 5.2 mm. This 216% change from baseline was significant (P <0.001). The mean width 4mmapical to the crest increased from 4.4 to 7.5 mm, a significant (P <0.001) 174% change. The histomorphometric analysis showed that the biopsy specimens consisted, on average, of 57% bone (36% graft material and 21% new bone) and 43% soft tissue and space. Conclusion: Our findings suggest that the LT PGA/TMC barrier membrane, used in conjunction with an allograft, provides lateral alveolar ridge augmentation comparable to that achieved with other materials without the necessity for bone-graft harvesting or a second procedure to remove the barrier membrane.