The importance of bronchial hyper-reactivity and allergy, and treatment with bronchodilators and corticosteroids, in the management of patients with cystic fibrosis is poorly understood. Three tests generally regarded as useful in the diagnosis of asthma were evaluated in 25 children with cystic fibrosis. The constancy of a child's response was assessed by histamine bronchial provocation, exercise challenge, and tests of skin allergy during a 6-month period. Although a positive response to these tests was related to impaired pulmonary function, 44% of children had variable responses to histamine, 56% to exercise, and 24% to skin tests which were unrelated to exacerbations of chest infection or to changes in pulmonary function. These results show the complex nature of airways hyper-reactivity and allergy in cystic fibrosis, and suggest that 'anti-asthma' therapy is not justified solely on the basis of one positive response to these tests.