Ten of fifteen cystic fibrosis children with positive skin prick tests to common antigens gave an immediate bronchial reaction to the antigen inhaled, five of them also gave a late reaction; however only one gave a history of asthma. The antigen most commonly eliciting a positive skin reaction in cystic fibrosis patients is Aspergillus fumigatus. In six children tested to this antigen the bronchial response varied, two were negative, one gave an immediate reaction and three gave a dual (immediate and late) reaction. None of the children showed the characteristic pattern of response to exercise seen in asthmatic patients, an initial rise in Peak Expiratory Flow Rate followed by a fall of greater than 14% below the resting level. Two patients showed an abnormal rise in Peak Expiratory Flow Rate during exercise, a pattern described previously in cystic fibrosis. The results suggest that bronchial allergy, immediate or late does completely explain susceptibility to asthma, and that other factors including perhaps the type of bronchial reactivity shown by bronchoconstriction after exercise may be required. However the majority of the children tested had bronchial allergy and anti-allergy therapy such as inhaled sodium cromoglycate may have a place in the management of selected patients with cystic fibrosis.