The bronchial reaction to isocapnic hyperpnoea in asthmatic children is described. There were two groups of children, asthmatic and non asthmatic and the following measurements were made: The vital capacity (CV). The maximum forced expired volume in one second FEV1 (VEMS). The airways resistance Raw (RVA). The Maximum expiratory flow at 75% of the VC, MEF75 (V75) before and after hyperventilating for one minute. In asthmatics the test leads to an increase in resistance (RVA) as well as a fall in FEV1 (VEMS) and V75. The fall in expiratory flow produced by isocapnic hyperpnoea is both a simple and a relevant test.