Purpose: The aim of this animal study was to evaluate, through immunochemical analysis, new bone formation in rabbit maxillary sinuses with and without bone grafts. Materials and Methods: Bilateral sinus augmentation procedures were performed in rabbits. Bony windows were created with a piezoelectric surgical device with a saw insert. In the first group, the bony window was repositioned after careful elevation of the sinus mucosa without bone grafting. A miniscrew was inserted into the bony window to support the elevated sinus membrane. In the second group, anorganic bovine graft (Bio-Oss) was grafted after very careful elevation of the sinus membrane. Collagen membrane was placed over the bone graft. Rabbits were sacrificed after 1, 2, 4, 6, and 8 weeks. The augmented sinuses were evaluated by immunochemical analysis of proliferating cell nuclear antigen (PCNA), type I collagen, and osteocalcin content. Results: Immunochemically positive cells for PCNA were present along the floor of the replaced bony window and the elevated sinus membrane from 1 to 4 weeks in the graftless group. In the grafted group, immunochemically positive cells for PCNA were not present after 1 week. In the ungrafted group, the expression of type I collagen was present from week 1. However, the expression of type I collagen was present after 2 weeks in the grafted group. Osteocalcin was observed in both groups after 1 week. Osteocalcin was observed along the floor of the replaced bony window in the graftless group but was not observed along the collagen membrane in the grafted group. Conclusion: According to this immunochemical study, faster and greater new bone formation was observed in sites that received no grafting material. The repositioned bony window may accelerate new bone formation earlier during healing versus the placement of a collagen membrane grafting material in the sinus. Schlagwörter: bone grafting, immunochemical study, maxillary sinus augmentation, new bone formation, replaceable bony window, sinus membrane elevation, sinus mucosa