The purpose of this study was a histomorphometric comparison of vital bone formation following maxillary sinus augmentation with two different particle sizes of anorganic bovine bone matrix (ABBM). Bilateral sinus floor augmentations were performed in 13 patients. Trephine bone cores were taken from the lateral window areas of 11 patients 6 to 8 months after augmentation for histologic and histomorphometric analysis. Bone samples from both the large and small particle size groups showed evidence of vital bone formation similar to that seen in previous studies, confirming the osteoconductivity of ABBM. Significant bone bridging was seen creating new trabeculae composed of the newly formed bone and residual ABBM particles. Histologic evaluation revealed the newly formed bone to be mostly woven bone with some remodeling to lamellar bone. Osteocytes were seen within the newly formed bone as well as osteoblast seams with recently formed osteoid. Isolated osteoclasts were observed on the ABBM surfaces. Vital bone formation (primary outcome measure) was more extensive in the large particle grafts compared with the small particle grafts (26.77% ± 9.63% vs 18.77% ± 4.74%, respectively). The histologic results reaffirm the osteoconductive ability of ABBM when used as the sole grafting material in maxillary sinus augmentation. The histomorphometric results at 6 to 8 months revealed a statistically significant increase (P = .02) in vital bone formation when the larger particle size was used. Additional studies should be performed to confirm these results.