Objective: The purpose of this study was to evaluate the effect of periapical lesion size on the degree of mucosal thickening of maxillary sinus and thickness of apical bone using cone-beam computed tomography. Materials and Methods: In Group 1 (teeth with apical lesions), diameter of apical lesion, width of apical bone, thickness of Schneiderian membrane; for Group 2 (teeth without apical lesions), width of apical bone in long axis of root and thickness of Schneiderian membrane were measured on coronal and sagittal images. Results: Mann-Whitney U-test revealed no significant difference between two groups regarding mucosal thickening and apical bone measurements (P > 0.05). Wilcoxon signed-rank test showed no significant difference between the measurements in sagittal and coronal slices (P > 0.05). Conclusion: Results of the present study showed that Schneiderian membrane near the maxillary premolars and molars with apical lesions is not significantly thicker compared to teeth without apical lesions.