PURPOSE: To examine the reaction of mucosa over time to a close contact with biomaterial after sinus elevation performed with deproteinized bovine bone mineral (DBBM) granules of two different sizes. MATERIALS AND METHODS: Bilateral sinus mucosa elevation was performed in 18 New Zealand rabbits through access on the nasal dorsum. DBBM with granule dimensions of either 1 to 2 mm (large group) or 0.250 to 1.0 mm (small group) were used to randomly fill the subantral hollow spaces. Biopsy specimens of the experimental sites from six animals in each group were obtained 2, 4, and 8 weeks after the surgery. RESULTS: The integrity of the sinus mucosa was clinically evaluated during surgery using a ×2.5 magnifying visual device. The sinus mucosa in contact with the biomaterial granules was found to be thinned compared with the pristine mucosa in all periods examined. Three hundred fifty-two thinned zones were found considering all 36 sinuses treated. Perforations of the sinus mucosa with extrusion of granules toward the sinus were observed, increasing in number over time. In the 8-week healing period, five perforations in three sinuses and eight perforations in four sinuses were found in the large and small groups, respectively. No differences were seen between the large and small groups. The differences between 2 and 8 weeks were statistically significant for the thinned mucosa width in both the large and the small groups and for the number of sinuses and perforations for the small group. CONCLUSION: Thinning zones and perforations of the sinus mucosa were seen increasing in number over time in regions in contact with graft granules in both the large and small groups.