Purpose: To radiographically monitor sinus membrane swelling after lateral sinus floor elevation surgery at short and long healing periods. Materials and Methods: For 26 patients seeking posterior maxillary implant-supported reconstruction, 32 lateral sinus floor elevations were performed using Piezosurgery. Sinus membranes were grafted using synthetic calcium phosphate bone substitutes, and graft volume was measured in cubic centimeters for each case. Cone beam computed tomography (CBCT) examination was conducted preoperatively in all patients and for each grafted sinus at 1 day (n = 8), 2 days (n = 9), 3 days (n = 8), or 7 days (n = 7) after surgery. Control CBCT was then performed for all patients at 3, 6, and 12 months after surgery. Sinus membrane thickness was measured on cross-sectional CBCT images at nine standardized points per sinus, before lateral sinus floor elevation and at all postoperative examinations. Results: Mean sinus membrane thickness was 0.73 mm before surgery, and 5 mm, 4.1 mm, 5.9 mm, and 7 mm, respectively, at 1, 2, 3, and 7 days after surgery. First week combined postoperative CBCT measurements of membrane thickness was 5.4 mm, then 1.3, 0.68, and 0.39 mm at 3, 6, and 12 months, respectively, after surgery. Membrane thickness significantly increased the first week after surgery and gradually decreased significantly at 3, 6, and 12 months in all groups (P .001). First-week postoperative measurements showed a significant increase in membrane thickness at 3 days compared with the 1- and 2-day results (P .001) and at 7 days compared with all other time points (P .001). Membrane thickness at 2 days did not change significantly compared with 1-day measurements. Larger graft volume was positively correlated with an increase in membrane thickness after surgery at all time points (n = 32; r = 0.527; P .001). Conclusion: After lateral sinus floor elevation surgery, transient swelling of sinus membrane is observed. It reaches a peak value 7 days after surgery and completely resolves over months. This swelling is correlated to the extent of sinus floor elevation. Schlagwörter: CBCT, lateral sinus floor elevation, mucosal thickness, posterior maxilla, sinus augmentation, sinus membrane