Objective: The extraction of an impacted mandibular third molar may result in periodontal complications on the distal surface of the adjacent second molar. The purpose of this study was to compare the influence of three full-thickness flaps on the periodontal healing of the adjacent second molar after extraction of impacted mandibular third molars. Method and Materials: Forty-five volunteers with bilateral impaction of the mandibular third molars were selected. Each patient was randomly assigned to one of three groups: group A (envelope flap modified by Thibauld and Parant), group B (Laskin triangular flap), and group C (envelope flap modified by Laskin). The periodontal health of the second molars was evaluated at 3, 6, 12, and 24 months after surgery via clinical measurements. Results: After 21 days, there was no correlation between postoperative complications (such as edema and alveolitis) and flap design. However, there was a statistically significant reduction of pocket probing depth (PPD) and increase of clinical attachment level (CAL) in group B compared to the other groups (P .05) 24 months after surgery. Conclusion: The effect of the type of flap used for mandibular third molar surgery on the periodontal status of the second molars as well as the factors that influence this outcome remains uncertain. Regardless of the flap design, the periodontal conditions of the adjacent second molar deteriorated after 12 and 24 months. The decision to use a certain type of flap should be based on the surgeon's preference. Keywords: complications, flap design, impacted teeth, pocket probing depth, third molar surgery