Congenital bilateral absence of the vas deferens (CBAVD) occurs in males when the tubes that carry sperm out of the testes (vas deferens) fail to develop properly. Although the testes usually develop and function normally, sperm cannot be transported through the vas deferens to become part of semen. As a result, men with this condition are unable to father children (infertile) unless they use assisted reproductive technologies. This condition has not been reported to affect sex drive or sexual performance. This condition can occur alone or as a sign of cystic fibrosis. Many men with CBAVD do not have the other characteristic features of cystic fibrosis; however, some men with this condition may experience mild respiratory or digestive problems.