Agitation exacerbates symptoms in patients with croup, but the reports on the therapeutic effects of sedation in these patients without intubation are scarce. We describe a typical case of croup wherein light sedation with dexmedetomidine was effective in treating and discuss how agitation exacerbates and sedation improves symptoms from the viewpoint of fluid dynamics theory in addition to the conventional explanation. The mechanism of dynamic airway collapse during inspiration in these patients supports the effectiveness of sedation with dexmedetomidine.