Upper airway reactivity was measured in 13 patients with obstructive sleep apnoea (OSA), using transient reflex laryngeal closure in response to dilute inhaled ammonia vapour. Upper airway reactivity was measured before and after 3 months of treatment with nasal continuous positive airway pressure (CPAP). Upper airway reactivity decreased significantly after treatment with nasal CPAP to values which were similar to those seen in normal subjects. We hypothesise that patients with OSA have increased upper airway reactivity, secondary to inflammation of the epithelial lining of the upper airway following the repeated injury of nocturnal airway obstruction, allowing the facilitated passage of inhaled irritants to the subepithelial receptors. Treatment of OSA with nasal CPAP may reverse these changes, although in the absence of a control group, these findings are provisional.