Ten adult patients with cystic fibrosis presented with lesions of the genital tract at different levels. Bilateral lesions of the vas deferens were constant and resulted in azoospermia in 9 cases; the fertile patient had a son. The sperm of these 10 patients was compared to that of 5 adult controls. A decrease in sperm volume, pH and fructose and an increase in citric acid and acid phosphatase were observed in the cystic fibrosis group. It is concluded that all subjects with azoospermia should be investigated for cystic fibrosis by means of the sweat test.