Introduction: Adhesive formulation may affect the marginal adaptation of composite resin restorations. Objectives: To evaluate the effect of adhesive filler content on the gingival marginal adaptation of Class II composite resin restorations. Materials and methods: Class II cavity preparations were made in ninety intact extracted human molars. Specimens were randomized and equally distributed in 6 groups (n = 15). Preparations were etched and All-Bond 3 primers A + B (Bisco, Schaumburg, IL/USA) were applied. One group received All-Bond 3 adhesive/resin (50 wt% filler) and the other five groups received the same resin composition but with filler contents of 0, 10, 20, 30 and 40 wt%. Teeth were restored with Filtek Supreme Ultra (3M, St-Paul, MN/USA), finished and stored in artificial saliva (37°C/24 h) before replicas were made for FE-SEM observation (200X). Quantitative margin analysis was performed based on a four criteria: MQ1 (continuous margin), MQ2 (marginal irregularities, porosities, roughness, no gap), MQ3 (small gaps of up to 2 microns) and MQ4 (severe gaps). Data were analyzed with a one-way ANOVA based on ranked data followed by the post-hoc Bonferroni test. Results: There was no statistically significant difference in "continuous margin" observed among the six filler contents (p = 0.0886). However, significantly less severe gaps were obtained with filler levels of 30 wt% and 40 wt% compared to 0 wt% (p = 0.0159). A noticeable but non-significant difference was observed for the unfilled adhesive (0 wt%), which showed the lowest mean % for continuous margins and the highest for severe gaps. Conclusion: Filler addition to adhesive appears to improve the marginal adaptation. Filler contents of 30 wt% and 40 wt% significantly reduced the occurrence of severe gaps compared to 0 wt%.