Cystic Fibrosis (CF) is one of the most frequent genetic diseases in the white populations of Europe and North America. Its clinical manifestations are highly variable, ranging from a characteristic life-shortening pathology of the lungs and the pancreas in classical CF to symptoms mainly restricted to male sterility in patients with congenital bilateral aplasia of the vasa deferentia (CBAVD). The genetic basis of CF is mutations in both copies of the CFTR gene, which codes for an ion channel. Even though one single mutation, deltaF508, is responsible for two-thirds of all mutated CFTR alleles, a total of over 1000 CFTR mutations have been described, whose relative frequencies vary between different ethnic groups, and whose biochemical consequences are correlated to the clinical manifestations of the disease. Since 4% of white Europeans and North Americans are healthy heterozygous carriers of CF, the molecular diagnosis of this disease, offered in the context of genetic counselling by a specialist in human genetics, is of great importance.