The aim of this study was to assess if the Hering-Breuer reflex could be provoked in young children. Thirty asthmatic children with a mean age of 5.3 years and 18 healthy children with a mean age of 7.0 years were studied. The presence of the reflex was indicated by prolongation of the spontaneous expiratory time following application of continuous positive airways pressure (CPAP). The occurrence of the reflex was then related to age, compliance of the respiratory system (Crs) measured by weighted spirometry, functional residual capacity (FRC) determined by helium gas dilution, and the change in lung volume resulting from the application of CPAP. The asthmatic children in whom the Hering-Breuer reflex was observed tended to be younger (P less than 0.01) and the reflex was present in those children of both groups who had more compliant lungs (P less than 0.05). The FRC was not different in asthmatic children with and without the Hering-Breuer reflex, but healthy children in whom the reflex was present tended to have smaller lung volumes. The change in lung volume resulting from application of CPAP was significantly greater in those children of both groups in whom the reflex was present (P less than 0.01 asthmatics, P less than 0.02 healthy children). We conclude that the Hering-Breuer reflex may be provoked in young children.