Congenital bilateral absence of the vas deferens (CBAVD) is responsible for 2-6% of male infertility in which mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene have been identified. To investigate CBAVD at the molecular level in Iran, we have characterized the mutations in the CFTR gene in 106 patients with this condition. None had clinical manifestations of cystic fibrosis (CF). We also analysed a DNA variant (the 5T allele) in a noncoding region of CFTR, which causes reduced levels of the normal CFTR protein and M470V exon 10 missense polymorphism. Five of the 106 patients with CBAVD had mutations in both copies of the CFTR gene, and none of them had the 5T allele. Eighty-five patients had a mutation in at least one copy of CFTR, and of these patients, 46 had one 5T allele (in 11 cases, two alleles and in 35 cases, just one allele of 5T was detected). In 21 patients, no CFTR and 5T mutations were found (19.81%). 5T/M470 genotype was found in 19 patients, 5T/V470 was found in 3 and 5T with heterozygote form of M470V was found in 24 CBAVD patients. In CBAVD patients, 28 F508del carriers were identified. Most of our patients with CBAVD have mutations in the CFTR gene. The combination of the 5T allele in one copy of the CFTR gene with a CF mutation in the other copy is the most common cause of CBAVD in Iran. The 5T allele mutation has a wide range of clinical presentations and revealed a high frequency, occurring in patients with CBAVD or moderate forms of CF and infertile men.