Objective: The aim of this retrospective study was to measure the sinus membrane's dimensional changes following maxillary sinus augmentation via a lateral approach, and to examine the variables affecting changes in the membrane's thickness. Method and Materials: Sixty-six sinuses corresponding to 50 patients (15 males and 35 females) who underwent lateral wall maxillary sinus augmentation (34 unilateral and 16 bilateral) were retrospectively evaluated. The sinus membrane thickness was measured on cross-sectional cone beam computed tomography (CBCT) scans which were performed prior to and 9 to 11 months' post maxillary sinus augmentation. The Wilcoxon signed-rank test and the Mann Whitney U test were both used to compare between baseline and postoperative sinus membrane thickness. Pearson correlation tests were used to analyze correlations between graft height and sinus membrane thickness changes. Results: The mean age was 53 ± 4 years (45 to 71 years). A total of 132 CBCT scans were analyzed pre- and postoperatively (n = 66). The mean thickness of the sinus membrane before the procedure was 2.61 ± 3.61 mm, while the mean thickness of the membrane after the procedure was 2.94 ± 3.51 (P > .20). Thin membranes at baseline ( 1.56 mm) thickened by a mean of 2.21 ± 2.34 mm, range −0.413 to 10.62 mm, (P .0001); thicker membranes (≥ 1.56 mm) lost 1.46 ± 3.96 mm thickness, range −7.8 to 9.31 mm (P .0001). A moderate negative correlation between the baseline membrane thickness and change in thickness was observed (P .0001, r = −.52). No correlation was found between the graft height and changes in the sinus membrane thickness. Conclusion: Lateral wall maxillary sinus augmentation seems to affect the sinus membrane thickness. These changes are associated with the preoperative thickness of the membrane. Keywords: CBCT, maxillary sinus augmentation, mucosal thickness, sinus membrane