Cystic fibrosis is one of the commonest genetic disorders with about 1 in 3000 newborns being affected. The molecular etiology of the disease is still not known. In spite of improved therapy the life expectancy is markedly reduced. Many parents of a child affected by CF request a prenatal diagnosis, when a further child is expected. By using molecular genetic methods the prenatal detection of CF is possible now with great accuracy in most families. This review describes the principles of the genetics and the diagnosis of the disease. Case reports illustrate risk figure ascertainment. Preconditions for performing the diagnosis in individual cases are outlined, and a guide to the practical approach is given.