Objective: To determine whether intravenous administration of dexamethasone during endoscopic sinus surgery in children will decrease scarring and edema during a second-look procedure. Design: Prospective, randomized, double-blind, placebo-controlled trial. Setting: University medical center. Patients: Forty-eight children undergoing endoscopic sinus surgery for chronic sinusitis. Intervention: Twenty-four children received intravenous dexamethasone and 24 received placebo intraoperatively before the start of the procedure. Main Outcome Measures: The status of the ethmoid cavity, the status of the mucosa in the maxillary sinuses, and the patency of the maxillary sinus ostium during the second-look procedure performed 2 to 3 weeks after the primary procedure. Results: Children who received intravenous dexamethasone had significantly less maxillary sinus mucosal edema, less ethmoid scarring, and a lower incidence of closure of the maxillary ostium (P=.02). During the second-look procedure, 62% of children in the noncortico-steroid group had abnormal findings vs 29% in the corticosteroid group. Patients with asthma, lower computed tomography scores, and no exposure to smoking had a significantly lower incidence of scarring with use of corticosteroids. Children older than 6 years benefited from intravenous corticosteroid therapy vs children 6 years and younger. Conclusions: Treatment with intravenous dexamethasone during endoscopic sinus surgery was safe and was helpful in reducing scarring and swelling noted during the second-look procedure. Use of corticosteroids was particularly helpful in children with asthma, lower computed tomography scores, and no exposure to smoking and in children older than 6 years.