One hundred and seventeen patients with cystic fibrosis (CF) were evaluated for criteria suggestive of allergic bronchopulmonary aspergillosis (ABPA) and atopy. We found positive skin tests to Aspergillus to be more common in patients with CF than comparison groups with asthma or non-specific nasal symptoms. This increased prick skin test reactivity to Aspergillus was especially noticeable in a group of CF patients having only one or two skin tests positive, preponderantly Aspergillus. Twelve of the CF patients (10%) had further immunologic criteria suggestive of ABPA. These patients had significantly worse lung function than the CF patients without such criteria.