Cystic fibrosis is an inherited multi-system disease, characterised by progressive lung disease and pancreatic insufficiency that is classically attributed to the dysfunction of a single gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR). The widely diverse phenotypic expression of CF is likely influenced by other genetic traits separate from the CFTR locus or modifier genes. Many of the genes currently under study as potential modifiers of CF, particularly those which influence the severity of lung disease, are involved in the control of infection, immunity and inflammation. Some of these include HLA class II antigens, mannose-binding lectin, alpha(1)-antitrypsin and alpha(1)-antichymotrypsin, glutathione-S-transferase, nitric oxide synthase type I, TNF-alpha, TGF-beta, IL-1beta and IL-1Ra.