About 10% of the cases of male infertility is represented by the obstruction of the seminal tract, which may be congenital or secondary to inflammatory events or surgery. The most frequent obstructive malformation of the seminal tract is the bilateral agenesia of the vas deferens. Such malformation is typical of the cystic fibrosis (CF), an autosomal recessive disorder determining chronic respiratory infections with bronchiectasia, and pancreatic failure. Recently the defective gene responsible for CF has been identified on the long arm of the chromosome 7. Congenital bilateral absence of the vas deferens (CBAVD) may be present in otherwise healthy males without clinical evidence of CF. Genetics studies demonstrated that most CBAVD display at least one detectable CF mutation, therefore this disease can be considered as an incomplete clinical form of CF. With the realization that a man with CBAVD may have CF, albeit a genital form, considerable care is required not only to document his specific mutations, but also to test his partner for CF mutations to evaluate the risk that their child would have CF. The association of chronic suppurating respiratory disease with obstructive azoospermia characterizes also the Young's syndrome. In this disease the obstruction could possibly be the result of defective epididymal sperm transport, related to an abnormality in the mucus. Despite some clinical common aspects, CF and Young's syndrome are two distinct entity. In fact, no CF mutations have been demonstrated in Young's syndrome. Congenital obstructive abnormalities of the vas deferens and epididymis are often associate to cryptorchidism (36-68% of the cases) and to patent processus vaginalis. The degree of testicular retention and processus vaginalis closure correlates well with the incidence of associated epididymal defects. Rare causes of congenital obstructive azoospermia are represent by the cyst of Müllerian or Wolffian origin. An obstruction to the progression of the sperm along the seminal tract can also be present in complex malformations, such as pseudohermaphroditism in which the infertility has a multifactorial etiology.