Objective: To compare the effectiveness of 4. and 8. mg prophylactic oral dexamethasone (PO) to control post-surgical swelling of impacted third molars. Material and methods: This randomized parallel-group clinical trial was conducted in the Dental Clinic of Universidad Privada Antenor Orrego (Trujillo - Peru), between August 2010 and March 2011. Patients who required extraction of impacted third molar were randomized into two groups: 27 received 4. mg dexamethasone PO, and 27 received 8. mg PO, according to established criteria. A standardized procedure was used, with effectiveness in controlling swelling by comparing pre- and post-operative facial measurements. Statistical analysis was performed using the t-test for comparison of means with a significance at 5%. Results: Greater effectiveness for controlling swelling was observed with 8. mg of dexamethasone than with 4. mg of dexamethasone. Patients in the 8. mg group only showed effectiveness in controlling swelling in the reference angle of the jaw-tragus (P=.500), while was not effective for the other references. The same results were observed in the 4. mg group (P=.163). Conclusion: Dexamethasone 8mg PO has greater effectiveness than 4mg to control postoperative edema of third molars. © 2013 SECOM.