CFTR (cystic fibrosis transmembrane conductance regulator) is a cAMP-regulated chloride (Cl(-)) channel that plays an important role in salt and fluid movement across epithelia. Cystic fibrosis (CF), the most common genetic disease among Caucasians, is caused by mutations in the gene encoding CFTR. The most predominant mutation, F508del, disturbs CFTR protein trafficking, resulting in a reduced number of CFTR in the plasma membrane. Recent studies indicate that CFTR is not only found in epithelia but also in human erythrocytes. Although considerable attempts have been made to quantify CFTR in cells, conclusions on numbers of CFTR molecules localized in the plasma membrane have been drawn indirectly. AFM has the power to provide the needed information, since both sub-molecular spatial resolution and direct protein recognition via antibody-antigen interaction can be observed. We performed a quantification study of the CFTR copies in erythrocyte membranes at the single molecule level, and compared the difference between healthy donors and CF patients. We detected that the number of CFTR molecules is reduced by 70% in erythrocytes of cystic fibrosis patients.