Background: Cryptorchidism represents the most common endocrine disease in boys, with infertility frequently observed in unilateral as well as bilateral forms. In this study, we examined the role of Ad (dark) spermatogonia for fertility in cryptorchid boys. The hypothesis to be proven was that boys lacking Ad spermatogonia will develop infertility despite a successful orchidopexy at an early age. Methods: To estimate total sperm count, one of the most predictive parameters of male fertility, we analyzed the ejaculatein 218 cryptorchid men and correlated it with the developmental stage of their germ cells at the time of successful surgery. Results: Abnormal sperm concentration (<40 × 10<sup>6</sup>/ejaculate) was found in half of the patients under the study. 47.5% of unilateral and 78% of bilateral cryptorchid males had their sperm concentration in the infertility range according to the WHO standards. If transformation into Ad spermatogonia had occured, age-related differences in the fertility outcome was observed. The younger the unilateral cryptorchid boys were at surgery, the higher their sperm count. Age-related difference was not found in the group of cryptorchid men having had no A dark spermatogonia at time of surgery, indicating that in this group a successful orchidopexy is insufficient to prevent infertility development and, in particular, the development of azoospermia. Conclusion: The presence of Ad spermatogonia at surgery is an excellent prognostic parameter for future fertility. Cryptorchid boys lacking these cells will develop infertility despite successful orchidopexy at an early age.