This randomized, split-mouth design study evaluated the adjunctive effect of allogeneic, freeze-dried, demineralized bone matrix on guided bone regeneration in a critical-size, supra-alveolar, peri-implant defect model. Cont ralateral supra-alveolar peri-implant defects, 5 mm in height, each including two titanium implants, were surgical ly created in five beagle dogs. Demineralized bone matrix in autologous blood eas placed over the implants in one randomly selected mandibular jaw quadrant. A s pace-making expanded-polytetrafluoroethylene membrane was used to provide guided bone regeneration bilaterall. Following a 16-week healing interval, tissue blocks were harvested and prepared for histometric analysis. Differences between experimental conditions (guided bone regen eration sites with and without demineralized bone) were evaluated using paired t tests (n = 4). Demineralized bone particles were discernible, with limited signs of resorption. The bone matrix particles appeared to be solidified within a dense connective tissue matrix and in close contact with the implants. Limited matrix remineralization was apparent adjacent to the alveolar crest. No statistically significant differeces were found between experimental conditions for any parameter examined. Peri-implant defect height averaged 5.0 ± 0.2 mm and 4.9 ± 0.4 mm, vertical bone regeneration 1.5 ± 0.9 mm and 1.1 ± .04 mm, osseointegration within the extent of the defect 10.0 ± 3.9% and 15.3 ± 5.3%, osseointeg ration within the extent of regenerated bone 30.4 ± 13.7% and 52.1 ± 17.9%, and osseointegration within the alveolar base 68.6 ± 13.1% and 74.4 ± 7.1% for guided bone sites with and without demineralized bone, respectively (P .05). The results suggest that freeze-dried demineralized bone has no adjunctive effect on guided bone regeneration in supra-alveolar peri-implant defects, that guided bone regeneration has a limited potential to enhance alveolar regeneration in this defect model, and that a 16-week healing interval appears insufficient for turnover and maturation of demineralized bone under provisions for guided bone regeneration.