Congenital bilateral absence of the vas deferens (CBAVD) is a frequent cause of obstructive azoospermia. Nearly 75% of men with CBAVD have at least 1 detectable common mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The different alleles at the (TG)(m)(T)(n) polymorphic locus at the 3' end of human CFTR intron 8 determine the efficiency of exon 9 splicing. To study the CFTR gene mutations and (TG)(m)(T)(n) polymorphisms in Iranian CBAVD patients with presumed low CF frequency and to better understand the complex regulation of exon 9 splicing among our study population, we analyzed CFTR mutations and (TG)(m)(T)(n) polymorphisms in 112 Iranian CBAVD, 7 congenital unilateral absence of the vas deferens males from Iran, and 84 fertile males as controls. Moreover, we compared the rate of CFTR transcripts with exon 9 (9+) with reduction of the (T)(n) repeat in our study population. Our study showed that the 5T mutation was present with high frequency in our patients. Longer (TG)(m) polymorphic tracts increase the proportion of exon 9 deletion transcripts but only when activated by the 5T allele. The combination of the 5T allele in 1 copy of the CFTR gene with a CF mutation in the other copy is the most common cause of CBAVD in the Iranian population. We also observed the highest level of exon 9+ splicing efficiency among the tested samples with the (TG)(12)(T)(7) allele, which represents the most common intron 8 splice variant allele in the general population. Our results support the idea that a putative role of the (T)(n) repeat is to distance the (TG)(m) repeat from the 3' splice site and that the different alleles at the (T)(n) locus affect the efficiency by which the splice acceptor consensus sequence is recognized.