Background Maxillary sinus floor augmentation (MSFA) with autologous bone and/or bone substitutes is a reliable procedure to develop an implant site. Loss of augmentation volume during the healing period seems to be dependent on the type of graft material and possibly on the sinus dimensions/morphology, e.g. a larger distance between the buccal and palatal sinus wall (i.e. “wide” sinus) might delay graft consolidation due to a longer distance angiogenic/osteogenic factors and cells need to cover to populate the graft. Aim/Hypothesis To assess the bucco-palatal width of the maxillary sinus in the premolar and molar region and any possible differences due to tooth region, gender, tooth-gap extent, and residual alveolar bone dimensions. Further, to attempt a rational, simple sinus classification based on its bucco-palatal width. Material and Methods CTs of the posterior maxilla were divided depending on tooth-gap extent into (a) edentulous, and (b) 1- or 2-tooth gap. Sites presenting on the CT >5 mm of the maxillary sinus, no previous augmentation procedures and/or oro-antral communication were included. A central orthoradial multiplanar reconstruction slice from each edentulous tooth site was chosen by a calibrated examiner to record the following parameters: (i) alveolar bone height (ABH), (ii) alveolar bone area (ABA), (iii) alveolar bone width (ABW) 2 mm apical to the alveolar crest (ABW2), (iv) ABW at the sinus floor (ABWS), (v) bucco-palatal sinus width (SW) and (vi) sinus area (SA) at a level 2, 4, 6, 8 and 10 mm above the sinus floor (SW2/SA2, SW4/SA4, etc.) (Figure 1). Based on mean values (i.e. <25th, 25–75th, >75th percentile and <33rd, 33–67th, >67th percentile) the sinus was classified as narrow, average, and wide, respectively, to attempt a simple sinus classification. Results Eighty-six partially dentate and 76 edentulous quadrants contributed with 383 tooth sites (58 first and 97 second premolars, 129 first and 99 second molars). Alveolar bone and sinus parameters did not differ depending on tooth-gap extent and gender, but differed significantly depending on tooth site, i.e. ABH and ABA was smallest at the first molar, and ABW, ABWS, SW, and SA were significantly larger at the molars. Further, ABW correlated significantly with SW and SA. High variability in SW among the various levels within the same tooth site, as well as high variability among tooth sites within the same person was observed, i.e. in >50% of the cases, sinus classification (i.e., narrow, average, or wide) varied depending on SW height level (Table 1) and among tooth sites within each patient. Conclusions and Clinical Implications Maxillary sinus bucco-palatal width varies significantly among the various height levels within the same tooth site and among tooth sites within the same person. A rational, simple classification of each sinus as narrow, average, or wide was not applicable.