Purpose: This study aimed to evaluate the effect of the placement of wide-diameter implants on bone stress concentrations and marginal bone loss in the first molar region. Study hypotheses held that increasing implant diameter would decrease peri-implant bone stress levels, but that statistically significant reductions in clinical bone loss would either (1) not be observed for any implant diameter or (2) be observed only for the widest implant diameter. Materials and Methods: Three-dimensional finite element analysis (3D FEA) was used to analyze the relationship between implant diameter and peri-implant bone thickness, cortical bone thickness, occlusal load direction, and percentage of boneto-implant contact on bone stress levels in the first molar region. A retrospective review of patient records was also conducted in three private practices to assess clinical outcomes and bone level changes around one implant design in three diameters (3.7, 4.7, and 6.0 mm) placed in first molar locations. Categorical variable summaries and comparisons of 3D FEA and clinical findings were made using the FREQ procedure, t test procedures (Student t tests, folded F tests, Satterthwaite t tests), and the NONPAR1WAY procedure (Wilcoxon nonparametric test). Results: Cumulative implant success was 98.4% and survival was 98% after a mean of 49.2 months. Although increasing implant diameter always reduced peri-implant stress concentrations in 3D FEA experiments, clinically, only 6.0-mm implants exhibited a statistically significant reduction in bone loss (0 mm) as compared to 3.7-mm and 4.7-mm implants. Conclusion: Only 6.0-mm implants were effective in reducing marginal bone loss in the first molar region. 3D FEA results supported previous clinical findings that maintaining approximately 1.8 mm of buccal plate thickness can help reduce bone stress concentrations and preserve buccal bone height. A history of periodontitis may adversely affect long-term marginal bone stability. Schlagwörter: bone, dental implant, implant diameter, molar, stress