The aim of this study is to introduce a new flap design in the surgical removal of impacted mandibular third molars a lingually based triangular flap and to compare this flap design with the routinely used triangular flap. This randomized, prospective, split-mouth study involved 22 patients with impacted bilateral mandibular third molars that were symmetrically positioned, mesially angulated, and retained in bone. The impacted teeth were removed in two sessions, using two different flap designs: the new alternative flap and the traditional triangular flap. Postoperative complications (pain, swelling, trismus, alveolar osteitis, and wound dehiscence) were recorded on days 2, 7, 14, and 21. The data obtained were analysed using the chi(2) test, the Mann Whitney U-test, and Pearson's correlation. In terms of the severity of postoperative facial swelling and trismus, there were no statistically significant differences between the flap designs (P > 0.05). The alternative flap exhibited higher pain scores at 12 h post-surgery (P < 0.05). In addition, the alternative flap group exhibited less wound dehiscence, although this was not statistically significant. Moreover, all wound dehiscence in this group occurred on sound bone. In conclusion, these results show that this new flap design is preferable to the routinely used flap for impacted third molar surgery.