Allergic bronchopulmonary aspergillosis (ABPA) results from a twofold mechanism: Th2-like hypersensitivity reaction and bronchial colonization by Aspergillus fumigatus. This relatively rare disease occurs in immunocompetent patients in two very different situations: refractory asthma and cystic fibrosis. Diagnosis in asthma patients is relatively easy; it is based on the association of several criteria: clinical (recurrent exacerbations despite adequate therapy and a positive A. fumigatus skin prick-test), laboratory (inconsistent blood eosinophilia, high serum levels of total IgE, presence of A. fumigatus-specific IgE and IgG) and radiological (mainly central bronchiectasis, sometimes transitory pulmonary infiltrates). Diagnosis is more difficult in patients with cystic fibrosis because of the similarity of their various criteria. Long-term prognosis is good in the early stages of the illness, although the natural history and course of the disease are not fully understood. Early diagnosis and active screening for exacerbations are recommended to prevent bronchiectasis and progression to end-stage lung disease. Two drugs have shown their efficacy in treating ABPA: corticosteroids and itraconazole. They are recommended in acute exacerbations and should not be used as long-term therapy, except in corticosteroid-dependent asthma and in some cases of cystic fibrosis.