A randomized, prospective, double-blind study was conducted to determine the efficacy of intravenous dexamethasone in reducing postoperative edema after bilateral sagittal split osteotomies of the mandible. Twenty-three patients were enrolled in the study and randomly assigned to one of the three groups. Each patient received one preoperative infusion and three postoperative infusions every 6 hours. Seven patients served as controls and received placebos for all infusions. Eight patients received dexamethasone, 16 mg preoperatively and three placebo postoperative doses. Eight patients received dexamethasone, 16 mg preoperatively and three 8-mg postoperative doses. Facial edema was quantified by computer scanning of standardized photographs. The underlying inflammatory process also was measured using C-reactive protein, erythrocyte sedimentation rate, and complete blood counts. Five sets of photographic and laboratory data were obtained for each patient: preoperative, day of surgery, and postoperative days 1, 2, and 3. Patients receiving dexamethasone demonstrated significantly less postoperative edema only on postoperative day 1 (P < .05) when measured photographically. C-reactive protein was significantly reduced on postoperative days 1, 2, and 3 (P < .05) in both dexamethasone groups. No significant difference was found between the two dexamethasone groups. Measurement of C-reactive protein seems to be the most sensitive method for comparing the effect of dexamethasone on postoperative inflammation. Preoperative intravenous dexamethasone significantly reduced postoperative inflammation and its associated edema after orthognathic surgery.