Allergic bronchopulmonary aspergillosis (ABPA) is a rare complication in patients with asthma but more common in patients with cystic fibrosis. In the presence of the fungus Aspergillus fumigatus (Af) in the lower respiratory tract, patients mount a heightened IgG and IgE humoral response specific for Af antigens. Studies on ABPA have suggested a pathogenic role for antigen specific CD4+ Th2 like T lymphocytes producing increased levels of IL-4 and IL-5. MHC class II genes coding for highly polymorphic HLA molecules have been shown to be the likely candidates for controlling immune responses to common allergens. However there has been a lack of information on the pathophysiological role of HLA genes in the development of ABPA. This review describes an association between HLA- class II alleles and the specific responses to Af antigen (Asp f 1) in ABPA. These studies focused on MHC restriction and distribution of HLA- class II alleles in two groups of unrelated North American Caucasian patients with cystic fibrosis and/or asthma. One group consisted of patients with a confirmed diagnosis of ABPA and a second group of patients with Af sensitivity but no ABPA. HLA association studies revealed that the predisposition to develop ABPA is associated with HLA-DR2 and DR5, and possibly DR4 or DR7. A strong association of HLA-DR antigens with ABPA reflects that HLA-DR molecules may present disease-causing peptides. On the other hand a significant association of HLA-DQ2 with Af sensitive nonABPA indicates the involvement of HLA-DQ molecules in protection. A combination of these genetic factors determines the outcome of ABPA in patients with cystic fibrosis and asthma.