Purpose: To analyze risk factors affecting sinus membrane perforation (SMP) during sinus floor elevation (SFE) procedures using the lateral window technique (LWT). Materials and Methods: For patients with SFEs using the LWT, patient-related risk factors (age/sex/smoking/diabetes) and surgical-anatomical–related risk factors (stage approach/sinus side/residual ridge height/sinus membrane thickness/previous surgical interventions) were compared between perforated and nonperforated sites and were evaluated for their influence affecting SMP. Additionally, SMPs were further subdivided into small/moderate ( 10 mm) or large (≥ 10 mm) in dimension, which were also analyzed for risk factors and consecutively for their influence on perforation. Results: The study sample comprised 434 SFE procedures in 355 patients; 94/355 patients (26.5%) presented SMP in 103 of 434 SFE procedures (23.8%). SFE procedures with (n = 103) and without (n = 331) SMP did not differ for patient-related risk factors but differed significantly (P = .001) for surgical-anatomical factors as follows: residual ridge height (3.05 ± 1.35 mm vs 4.15 ± 1.46 mm), sinus membrane thickness (1.2 ± 0.5 mm vs 2.6 ± 1.1 mm), prevalence of staged procedures (78.6% vs 57.7%), presence of maxillary sinus septa (75.7% vs 14.2%), presence of thin ( 1.5 mm) mucosa biotype (62.1% vs 29%), and previous oral surgical interventions (37.9% vs 16.3%). In the multivariate analysis, significant associations of SMP were found with the presence of sinus septa (odds ratio [OR] = 31.992; P = .001), residual ridge height (OR = 1.563; P = .007), sinus membrane thickness (OR = 1.057; P = .001), presence of thin ( 1.5 mm) sinus biotype (OR = 8.883; P = .001), previous surgical interventions (OR = 4.689; P = .002), and smoking habits (OR = 2.238; P = .030). For inducing a large (≥ 10 mm) SMP, the presence of thin sinus membrane thickness/thin sinus membrane biotype (OR = 5.319; P = .006; OR = 22.222; P = .001) and reduced alveolar ridge height (OR = 0.629; P = .026) were assessed as being significant risk factors. Conclusion: In general, the presence of sinus septa, thin sinus mucosa, staged procedures, and previous surgical interventions are the main risk factors inducing SMP for SFE using the LWT. In particular, the presence of thin sinus membrane in conjunction with a staged procedure significantly increases the risk for a large SMP. Schlagwörter: lateral window, membrane perforation, perforation size, risk factors, sinus elevation