Infectious croup is a common and an important cause of upper airway obstruction in young children. Despite its frequency and potentially serious nature, there is still no definite conclusion regarding the beneficial effect of corticosteroid. A randomized controlled study on the effects of dexamethasone in infectious croup was conducted at the Department of Pediatrics, Ramathibodi Hospital between January 1985 and September 1986. Thirty-two patients, 2-37 months old, were included in this study. Fourteen patients received dexamethasone (0.5 mg/kg/dose daily for 3 days) and eighteen patients were the control group. The dexamethasone group had significantly lower croup scores at 48 hour (p < 0.05), shorter hospital course (p < 0.005) and lower incidence of endotracheal intubation (p < 0.05) than the control group. Five patients in the control group required endotracheal intubation. Complications included four episodes of pneumonia, one episode of sepsis, and one bacterial tracheitis. Pneumonia and sepsis occurred only in the control group. We concluded that dexamethasone therapy decrease the severity of infectious croup and the risk of complications.