Cystic fibrosis (CF), the most common life-limiting single gene disorder, is caused by mutations to the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR is expressed by epithelial cells and immune cells, including APCs, monocytes and lymphocytes. Individuals with CF appear to have a functioning immune system although they are unable to eradicate respiratory infections. Previous studies on mouse CFTR-negative lymphocytes have shown abnormal immune responses to antigen. We hypothesized it would be possible to detect the presence of inherent immune regulatory defects by investigation of CD4+ lymphocyte subsets in peripheral blood of CF patients, and that the incorporation of clinical parameters into this analysis would allow identification of biomarkers for prediction of worsened clinical state. Peripheral blood mononuclear cells (PBMC) were isolated from adults and children with CF and healthy age-matched controls, then stimulated with Ca2+ionophore and phorbol myristate acetate (PMA). PBMC were stained with the relevant antibodies for multi-colour flow cytometry analysis of the Th1, Th2, Th17, FoxP3+ Treg, IL-10-producing Tr1 and TGF-β-producing Th3 CD4+ subsets. Investigation of the subsets revealed there to be a higher level of the regulatory subsets, FoxP3+ Treg, IL-10-producing Tr1 and TGF-β-producing Th3 cells, in CF adults compared with healthy controls but no difference between healthy and CF children. No significant difference was found in the effector subsets, Th1, Th2 and Th17 in either group. As differences were seen only in adults, this suggests that regulatory subsets are increased because of chronic infection and inflammation. Most interestingly a strong inverse correlation was found between percentage of Th17 cells and lung function (FEV1% predicted), a relationship that was present in both the CF adults and children. A similar but weaker relationship was seen between FoxP3+ Treg cells and lung function, but was only present in CF adults. This may indicate that Th17 cells, and to a lesser extent FoxP3+ Treg cells, could be a useful marker for lung function decline in CF.