Allergic bronchopulmonary aspergillosis (ABPA) is an important complication of cystic fibrosis. It is a hypersensitivity reaction to Aspergillus fumigatus, leading to a Th2 CD4 response mediated by the release of specific IgE. If ABPA is not treated early, it can cause severe impairment in lung function and long-term lung damage. Hence, early recognition with a prompt diagnosis is important. Due to clinical and radiological features of ABPA overlapping with those of bacterial or viral pulmonary exacerbations in cystic fibrosis, diagnosis can sometimes be difficult. Specific criteria for making the diagnosis of ABPA have been suggested. Newer serological tests, such as specific IgE to recombinant allergens and the detection of thymus- and activation-regulated chemokine, are being developed to improve early detection and monitoring of ABPA with greater sensitivity and specificity.