An increasing incidence of allergic bronchopulmonary aspergillosis (ABPA) as a complication in patients with cystic fibrosis (CF) is reported. The objective of this retrospective case-control study was to assess potential risk factors for ABPA and for Aspergillus fumigatus sensitisation (AFS). In a group of 160 CF patients, 11 (7% ) fulfilled the diagnostic criteria for ABPA and 20 (13% ) had evidence of AFS. They were compared to 62 control CF patients (25 for ABPA and 37 for AFS group) without evidence of ABPA or AFS using extended matching for sex, age and weight. AFS patients had received significantly higher cumulative doses of inhaled corticosteroids than their respective controls (OR 8.0; 95% CI 1.74-63). Bronchial colonisation with Stenotropho monasmaltophilia was strongly and independently associated with ABPA (OR 20; 95% CI 2.8-infinity). A longer duration of Pseudomonas aeruginosa colonisation was independently associated with AFS (OR per year 1.50; 95% CI 1.12-infinity). Conclusion: Cystic fibrosis patients with allergic bronchopulmonary aspergillosis have a more frequent isolation of S. maltophilia in their sputum than their controls. Longer duration of colonisation with P. aeruginosa is a risk factor for Aspergillus fumigatus sensitisation. Higher cumulative doses of inhaled corticosteroids are associated with Aspergillus fumigatus sensitisation and their role as a risk factor needs to be clarified. An increasing incidence of allergic bronchopulmonary aspergillosis (ABPA) as a complication in patients with cystic fibrosis (CF) is reported. The objective of this retrospective case-control study was to assess potential risk factors for ABPA and for Aspergillus fumigatus sensitisation (AFS). They were compared to 62 control CF patients (25 for ABPA and 37 for AFS group) without evidence of ABPA or AFS using extended matching for sex, age and weight. AFS patients had received significantly higher cumulative doses of inhaled corticosteroids than their respective controls (OR 8.0; 95% CI 1.74-63). Bronchial colonization with Stenotropho monasmaltophilia was strongly and independently associated with ABPA (OR 20; 95% CI 2.8-infinity). A longer duration of Pseudomonas aeruginosa colonisation was associated associated with AFS (OR per year 1.50; 95% CI 1.12-infinity). Co nclusion: Cystic fibrosis patients with allergic bronchopulmonary aspergillosis have a more frequent isolation of S. maltophilia in their sputum than their controls. Longer duration of colonization with P. aeruginosa is a risk factor for Aspergillus fumigatus sensitisation. Higher cumulative doses of inhaled corticosteroids are associated with Aspergillus fumigatus sensitisation and their role as a risk factor needs to be clarified.