Objectives: To test whether a reduction of bone window dimension, in a split-mouth randomized study design, focused on lateral sinus floor elevations, can achieve better results than a wider window in terms of augmented bone height and a reduction of patient discomfort and surgical complications. Materials and methods: Of the sixteen subjects enrolled in the study, each underwent a bilateral sinus lift procedure based on two different access flaps to maxillary sinus. Test side: small access window (6 × 6 mm) + bone filling using a special device. Control side: large access window (10 × 8 mm) + manual bone filling. Alveolar bone height and width were measured at pre-op and 6-month post-op CT scans; repeatable measurements were obtained using radiographic stents. Surgical intervention duration was also recorded. Patients' evaluation of surgical discomfort was assessed using a VAS diagram at 7-day, 14-day and 30-day follow-up. Results: A significant bone augmentation in height and width of alveolar crest was obtained in both test (8.71 ± 1.11 mm, 4.70 ± 0.58 mm) and control (8.5 ± 2.02 mm, 4.68 ± 0.70 mm) sides, although no significant differences were found between the two groups. Neither any significant differences emerge in data concerning the duration of the intervention (Test 42.62 ± 6.67 min, Control 41.68 ± 8.34 min). Patients' opinion relating to surgical discomfort showed a preference for test procedure at 7-day, 14-day and 30-day follow-up. Conclusions: A reduction of window dimensions did not affect the safety of the surgical procedure. The two testing techniques showed no statistically significant differences in surgical intervention duration. Patients' opinion at 7-day and 14-day post-op showed a preference for test procedure. © 2016 John Wiley & Sons A/S.