Purpose: In boys with cryptorchidism older than 2 years a testicular biopsy at time of orchiopexy shows lack of germ cells in 10–40% of the cases. The number of spermatogonia per tubule is prognostic for subsequent fertility potential. A biopsy without germ cells is associated with 33–100% risk of infertility. In order to increase the number of germ cells, and thereby the fertility potential, additional hormonal therapy has been attempted before surgery. In a study, small doses of the gonadotropin-releasing hormone analogue buserelin before orchiopexy caused higher values. Others have found that hormonal treatment with human chorionic gonadotropin or gonadotropin releasing hormone analogue may harm the germ cells in cryptorchidism. The aim of the study is to demonstrate that additional hormonal therapy with erythropoietin has a positive effect on the number of germ cells. Materials and Methods: Erythropoietin (Eprex) 100 IU/kg were administered subcutaneously weekly for 3 months prior to surgery in two cryptorchid boys, 6 months old and 1 year 9 months old, respectively, with renal function impairment. Results: The number of spermatogonia per tubular cross-section in testicular biopsies was unusually high in both erythropoietin- treated cryptorchid cases compared to the control material of biopsies from the undescended testes of 698 cryptorchid patients and compared to the normal values. Conclusion: There are several hypothetic mechanisms that can explain the elevated number of spermatogonia seen in our erythropoietin treated cryptorchid patients. Erythropoietin may have a positive effect on germ cell proliferation in cryptorchidism.