Genetic diseases presenting with different phenotypes are generally classified as distinct disorders before their molecular defect is revealed, as exemplified by the recent advance in understanding of the molecular biology of cystic fibrosis and an obstructive form of infertility, known as congenital absence of the vas deferens. The majority of men with congenital absence of the vas deferens have a defect in both copies of the CFTR gene and therefore represent a distinct phenotypic form of cystic fibrosis. These developments help us to gain new insight into the genetic basis of phenotypic variability and the possible contributing mechanisms in cystic fibrosis. Some of the lessons learned from the relationship between cystic fibrosis and congenital absence of the vas deferens may be useful in the understanding of other genetic disorders.