Objectives: The aim of this study was to evaluate the effect of the use of deproteinized bovine bone as a filler material of bony defects derived from surgical excision of maxillary and mandibular cysts having a diameter >= 20 mm. Methods: The study was approved by the local ethics committee. 19 patients requiring the removal of maxillary cyst >= 20mm diameter were considered. Patients were randomly divided into 2 groups: the bone defect was filled with granules of deproteinized bovine bone (group 1); no filling material (group 2). Clinical and radiological follow up was performed for every patient. Clinical follow up was scheduled as follows: 7 days, 1, 6 and 12 months after surgery. A computerized method was adopted to compare the pre-op and the 1-year post-op CT scans. Using an in-house software, cysts and residual radiolucent areas, corresponding to pre operatory CT and to post-operatory CT respectively, were manually outlined on single axial layers. The volume of each cyst was computed as the sum of the voxel included in the segmentation. Results: Results are preliminary, since not all the 19 patients enrolled for this study completed the 1-year follow up. 8 patients were considered. The comparison between the pre-operatory and the post-operatory CT revealed that the average volume of residual radiolucency in group 1 was reduced with respect to group 2 (2.26% vs 27,87%). Conclusions: The use of deproteinized bovine bone seems to influence positively the healing of alveolar bone critical size defects in humans. Keywords: bioss, alveolar bone defect, maxillary cyst, deproteinized bovine bone