Purpose: The aim of this study was to evaluate the effect of two different concentrations (4 and 8 mg) of dexamethasone to decrease the swelling and trismus after the surgical extraction of mandibular impacted third molars. Methods: This randomized clinical trial comprised thirty (30) adult patients of both genders with no local or systemic problems, with bilateral impacted lower third molars in similar position, where surgical extraction had been indicated. They were given 4 mg and 8 mg of dexamethasone 1 hour before the surgical procedure at the first or second surgery. The choice of which side to operate first and the amount of concentration of medication to use was made randomly and double-blindly. Postoperative pain was evaluated using a visual analog scale (VAS) and the degree of swelling was evaluated through facial reference points? variation. The presence of trismus was analyzed through measurement of the interincisal distance (IID). These assessments were obtained before the operation and 24h and 48h after the surgery. Results: Based on statistic analysis (pared t-student and Wilcoxon tests), the results showed a significant difference in the measurements of the degree of swelling and trismus of the treated sample. 8 mg of dexamethasone promoted a greater reduction of symptoms than 4mg of dexamethasone Conclusions: The administration of 8 mg of the dexamethasone was more effective than 4mg of the dexamethasone to reduce the degree of swelling and trismus. However, it had no effect on pain control.