Cystic fibrosis (CF) is the most common life-limiting single gene disorder. It occurs due to mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene that 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. We hypothesized that investigation of CD4+ T cells in CF carriers would allow for detection of an inherent defect in immune regulation. CF carriers have one defective copy of the CFTR gene but display no obvious clinical symptoms. This would allow for investigation of T cell subsets without the interfering influence of chronic respiratory infection and inflammation that is seen in those with the disease. Peripheral blood mononuclear cells (PBMC) were isolated from CF carriers and healthy aged 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. Analysis of these subsets revealed there to be increased levels of the FoxP3+ Treg and IL-10-producing Tr1 cells in CF carriers compared to healthy controls although no difference in the TGF-β-producing Th3 cells. No significant differences were found in the Th1, Th2 and Th17 effector subsets. These findings are mirrored in our previous analyses involving CF patients. While CF patients were additionally found to have increased TGF-β-producing Th3 cells this was an increase significantly correlated with Pseudomonas aeruginosa infection. This may suggest that high levels of the FoxP3+ Treg and IL-10-producing Tr1 cells are an inherent characteristic of the CFTR mutation.