Cystic fibrosis (CF) children had a greater variability of skin prick test response compared with asthmatic children, when the tests were performed on two occasions. This suggests that there is a different cause for the allergy in the two groups, perhaps because the asthmatics have a transient period of susceptibility to sensitization in infancy and the CF children a persistent vulnerability. Amongst the asthmatic children, clinical improvement was associated with loss of some skin prick test reactions, and clinical deterioration with an increase in the number of positive reactions. Thus continuing allergic reactions may maintain a mucosal defect, resulting in a persistence of susceptibility to allergen sensitization, and control of the reactions may re-establish normal mucosal defence mechanisms.