Infertility is a common symptom of cystic fibrosis, especially in men (95% become sterile). It is caused by blockage of the vas deferens and the epididymis, which result in degeneration of the tubules. The purpose of this study was to verify the frequency of CFTR gene mutation in patients with bilateral agenesis of the vas deferens using SSCP and sequencing. The study population consisted of 40 white individuals with agenesis of the vas deferens as well as their 12 siblings without agenesis of the vas deferens. CTFR gene mutation was found in 22 of the 40 patients (55%) and it was possible to detect both mutating alleles in these 22 patients. The most frequent genotype found was ?F508/IVS8-5T. There was no genotype concordance in siblings. Our results show the importance of the investigation of CFTR mutation in patients with vas deferens agenesis.