Purpose: The objective of the present systematic review was to compare the postoperative outcomes between envelope and triangular flaps after mandibular third molar surgery. Materials and Methods: The PubMed, Embase, Web of Science, and Cochrane Library databases were systematically searched from the inception date to November 2018. Randomized and nonrandomized controlled trials were included if they had met certain inclusion and exclusion criteria. The predictor variable was the flap design, envelope or triangular. The outcome variables were operation time, pain, trismus, alveolar osteitis (AO) incidence, wound dehiscence, and swelling. The methodologic quality assessment was performed in accordance with the Cochrane Collaboration guidelines. The metaanalysis was performed using Review Manager, version 5.2. Results: A total of 21 studieswere included for qualitative synthesis, 18 of which were included in themetaanalysis. The use of envelope flaps required less operation time than triangular flaps (P<.00001; I-2 = 9%). Inthe Pell and Gregory A and B subgroups, envelope flaps were associated with a significantly lower visual analog scale score at 3 (P =.05, I-2 = 0%) and 7 (P =.02; I-2 = 0%) postoperative days and with a greater postoperative interincisal distance at 7postoperative days (P=.04; I-2 = 0%). In contrast, envelope flaps were associated with a greater AO incidence in the subgroup of split-mouth randomized controlled trials (P =.001; I-2 = 0%). Conclusions: Envelope flaps required a shorter operation time than triangular flaps and were associated with less postoperative pain and trismus when applied to impacted mandibular third molars of Pell and Gregory Class A or B. In contrast, triangular flaps were associated with a decreased incidence of AO compared with envelope flaps. (C) 2019 American Association of Oral and Maxillofacial Surgeons.