Thirty-one patients with cystic fibrosis (CF) were thoroughly evaluated for allergy. This included a clinical history, skin tests with twenty-three allergens and bronchial provocation with inhaled allergens and histamine. The bronchial response was measured by whole body plethysmography. Of the patients studied, 40% 0 showed a bronchoconstrictor response to inhaled allergens, despite the fact that none had reported asthma in their clinical history. Strong skin test reactions (3+ and 4+) and weak reactions (2+) were associated with 65% and 4% of positive reactions of the airways respectively. Weak skin reactions with Aspergillus fumigatus, however, were associated with 43% of positive bronchial challenges. In addition to Aspergillus, the mould Alternaria tenuis was found to be an important allergen causing a bronchial response in CF patients. There was no correlation between the thresholds of bronchial sensitivity to allergen and histamine. suggesting that the pathogenetic mechanisms of CF and bronchial asthma are different.