This prospective study conducted on 24 children, who have now reached adulthood, was undertaken to determine the value of testicular biopsies in cryptorchidism relative to fertility prognosis. A significant correlation between the number of germ cells in the biopsy and sperm count was found to exist (p less than 5%). Thus, the testicular biopsy performed during orchiopexy was found to have a prognostic value. The hormonal findings (LH, FSH) suggest that in cryptorchid adults impaired Leydig cell function exists only when serious tubular damage is present. Because of its prognostic value with respect to fertility and the probability of discovering carcinoma in situ cells, it is strongly recommended that a testicular biopsy is performed in cryptorchid boys.