IntroductionThird molar surgery is almost one of the most frequent procedures performed by maxillofacial surgeons. The surgical trauma leads to post-operation complications such as pain, inflammation and trismus.1 About 3-5 hours following surgery, the pain reaches its maximum intensity and may last 2-3 days; and then diminishes within 7 days after surgery.2,3 Moreover, post-operative inflammation disappears 5-7 days after surgery. 4 It has been recommended to use local or systemic steroid and non-steroid anti-inflammatory therapy to reduce inflammation and relieve pain after molar surgery, but these drugs present some side effects, including gastrointestinal issues, systemic bleeding and allergic reactions.5 Several studies have demonstrated that laser therapy can accelerate cell and tissue reconstruction as well as relieve post-operative pain. 5,6 Laser therapy is an open research field. However, some studies have shown useful findings in treatment of dentin hypersensitivity, temporomandibular joint disorders, inferior alveolar nerve paraesthesia resulting from third molar surgery, sagittal osteotomy, trigeminal neuralgia, labial herpes, aphthous ulcers and post-chemotherapy and ray inflammation. [7][8][9][10][11][12] As there are conflicting findings regarding the effectiveness of low-level laser therapy (LLLT) and the importance of controlling third molar surgery's complications, this study aimed to assess the effect of LLLT on pain, swelling and maximum mouth opening in patients undergoing third molar surgery. In this study, we increased the number and variety of studied groups in order to reduce the error rate, in comparison to former researches. To eliminate the possible differences between several people in relevance to pain amount and also their reports on it, a bilateral surgery group was used to study the effects of LLLT on the post-operation pain and swelling. On the other hand, as maximum mouth The aim of this study was to assess the effect of low-level laser therapy (LLLT) on pain, swelling and maximum mouth opening in patients undergoing third molar surgery. Methods: A prospective, randomized double-blind study was undertaken on 44 patients at the Dental School, Ahvaz Jundishapur University of Medical Sciences, in 2015. A lowlevel laser was randomly applied on one of the two sides after surgery of 15 patients. The experimental side received 18 J/cm 2 of energy density, wavelength of 980 nm, and output power of 1.8 W. On the control side, a hand-piece was applied intra-orally, but laser was not activated. In addition, in order to evaluate trismus, 13 patients were treated by unilateral laser therapy and 16 patients did not receive laser therapy at all. The laser was administered intraorally on two points of vestibular and lingual sides at 1 cm from the surgery site, and extraorally at the emergence of the masseter muscle, immediately after surgery, and repeated 24 hours later. The pain, swelling and maximum mouth opening (MMO) were compared between the two groups at 24 hours and a week after surger...