Purpose: To test a minimally invasive flap in the lateral approach for maxillary sinus floor elevation when compared to a trapezoidal flap. Materials and methods: Each patient received a bilateral sinus elevation procedure based on two different randomly allocated surgical approaches according to a split-mouth design: in the test side, a horizontal incision at mucogingival line was realised; in the control side, a trapezoidal flap was elevated to prepare the lateral window. Each sinus was filled using deproteinised bovine bone and the window covered with a collagen membrane. Implants were inserted according to a two-stage technique 6 months after sinus elevation procedures, submerged and then loaded after 6 months with definitive screw-retained metal-ceramic prostheses. Patients were followed up to 4 months post-loading. Outcome measures included: implant and prosthesis failures, complications, peri-implant marginal bone level changes, residual bone height and width, recorded before sinus augmentation and 180 days post-intervention by computed tomography (CT) scans, surgical times and patient discomfort data, assessed using a visual analogue scale (VAS) diagram for each treated side at 1, 7, 14 and 30 days of follow-up. Results: Seventeen patients were enrolled in this trial and none dropped out. Five completely edentulous patients were treated with full-arch prosthetic restorations (Toronto dental prosthesis) and 12 patients with partial metal-ceramic screw-retained prostheses. In total 72 implants were inserted, 37 in the test group and 35 in the control group. No prosthesis or implant failures occurred. A total of seven patients showed complications: five of them had complications in both test and control sides. Eight complications were detected in each group. During maxillary sinus elevation procedures, nine interventions (four from the test group and five from the control group) were affected by intrasurgical complications (six membrane perforations and three severe bleedings); post-surgical complications occurred to three patients (one submucosal emphysema in a patient from the test group, one wound dehiscence and a graft infection both recorded in the control group); a total of four implants in two patients in the test group, versus three implants from one patient in the control group showed peri-implant mucositis. There were no statistically significant differences in complications between the two groups (P = 1.00; 95% confidence interval [CI]: 0.15-3.11). Patients from the control group, at 4 months after loading, lost on average 0.53 mm (standard deviation [SD] 0.27; 95% CI: 0.40-0.65), and patients from the test group lost on average 0.66 mm (SD 0.27; 95% CI: 0.53-0.78); the difference was not statistically significant (mean difference: 0.07 mm; SD 0.34; 95% CI: −0.03-0.17; P = 0.102). Significant values for bone augmentation in height, 9.26 mm (SD 1.46; 95% CI: 8.56-9.95) in the test group and 9.38 mm (SD 1.95; 95% CI: 8.45-10.30) in the control group, and width, 1.68 mm (SD 1.04; 95% CI: 1.18-2.20) in the test group and 1.60 mm (SD 1.27; 95% CI: 0.99-2.20) in the control group, were found from the 180-day CT scans. No statistically significant differences were detected between the two groups, either for the bone augmentation data (difference: 0.27 mm; 95% CI: 0.15-0.38; P = 0.60) or for the bone width values (difference: 0.02 mm; 95% CI: 0.07-0.11; P = 0.67). A significant reduction in the total surgical time was found in the test group (mean difference: 6.64 minutes; SD 4.32; 95% CI: 4.58-8.69), with the difference being statistically significant (P = 0.009). Evaluation of patients' postoperative discomfort showed a significant preference for the test procedure: at 1 day the VAS value was 4 (interquartile range [IQR] 2 to 5) in the test group; in the control group the VAS score was 5 (IQR 4 to 8). The intergroup difference was statistically significant (P = 0.002). At the 7-day follow-up, the VAS value was 1 in the test group (IQR 0 to 3) and 3 in the control group (IQR 0 to 7), this difference was statistically significant (P = 0.003). No differences were detected at 14 and 30 days (P > 0.05). Conclusions: A minimally invasive approach to access the sinus cavity can be as successful as a conventional trapezoidal flap in maxillary sinus floor elevations.