In order to determine the incidence of allergic bronchopulmonary aspergillosis (ABPA) in patients with cystic fibrosis (CF), we reviewed the records of 236 patients followed up at the Duke CF Center. Sixty patients (25 percent) had colonies of Aspergillus fumigatus. These patients were older and had more severe disease as assessed by lower Shwachman-Kulczycki (S-K) scores than the patients who did not have evidence of A fumigatus. In 15 of the patients with A fumigatus (6.5 percent of the total population), the diagnosis was ABPA. Age and S-K scores were not significantly different from those of the patients with A fumigatus without ABPA. Diagnostic features of the affected patients included wheezing refractory to bronchodilator therapy, persistent pulmonary infiltrates, peripheral eosinophilia, positive skin reactivity to an A fumigatus antigen and elevated total serum IgE levels. Steroid therapy was started for all patients, and clinical improvement was noted within 1 month as evidenced by decreased symptoms and weight gain. Chest x-ray films usually showed improvement. Vital capacity improved in all but two patients. Total IgE did not consistently decrease in response to therapy. Although the diagnosis of ABPA may be difficult to establish, ABPA commonly is associated with CF. Most patients respond to steroid therapy; however, the effect of therapy on the course of the disease is difficult to assess.