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This study was undertaken to evaluate the effect of hyperbaric oxygen (HBO) on the repair of critical-sized defects in the presence and absence of a nonvascularized autogenous bone graft.Ten New Zealand White rabbits were randomly divided into 2 groups of 5 animals each. Bilateral 15-mm calvarial defects were created in the parietal bones of each animal, resulting in 20 critical-sized defects. Autogenous bone grafts (ABG) were allocated to the left or right defect of each animal. Group 1 received HBO treatment at 2.4 ATA 100% oxygen for 90 minutes per day 5 days a week for 4 weeks. Group 2 served as a normobaric (NBO) control, breathing only room air. The animals in each group were humanely killed at 6 weeks. Calvaria were analyzed by micro-CT and histomorphometry.Micro-CT analysis indicated that as expected there was a higher bone mineral density (BMD) and bone mineral content (BMC) in ABG than unfilled defects (P < .05). However, there was a significant decline in the bone mineral content (BMC) of HBO-treated grafted defects compared to NBO-treated grafted defects (P < .05). Histologically complete bridging of the defect was observed in both NBO and HBO ABG grafted defects. Histomorphometic analysis showed that HBO treatment increased new bone and marrow, and reduced fibrous tissue in the defects (P < .01 for all). Examination of residual graft showed a near significant reduction in residual graft volume (11.2 ± 4.7 versus 19.1 ± 7.7, HBO versus NBO P = .085) in the HBO group. The use of a graft increased new bone and marrow in the NBO group (P < .001 for both); however, in the HBO-treated animals the differences between grafted and ungrafted were not significant.HBO enhances bony healing in ungrafted rabbit calvarial critical-sized defects and may increase the rate of residual graft resorption in autogenous bone-grafted defects.