The testis which remains nonscrotal beyond puberty will not produce sperm. While much of the data presented imply that therapy of cryptorchidism during childhood will decrease the likelihood of infertility, the available data do not substantiate this. There are also inadequate data to indicate whether treatment at very young ages in childhood decreases the risk of infertility. In fact, it is not clear that fertility rates among males who were unilaterally cryptorchid is different than that of the unaffected male population! Therefore, verification is needed to determine whether or not fertility is decreased in cryptorchidism. Paternity is a better index for verification than sperm counts since it is known that men with subnormal sperm counts may have normal paternity rates. Such verification using paternity may require more than fathering a child, such as the age of the father at the birth of the first child, length of marriage or partnership before birth of the first or subsequent children, or duration of intercourse without contraception to birth. Because of the multiple etiologies of the cryptorchid state, factors such as the relative size of the testis before treatment, and the position of the testis and the histology of the testis need to be considered since the small testis, the abdominal testis and the testis with the most histologic changes would appear to be at the greatest risk for defects in spermatogenesis. Until these data become available, most physicians are likely to recommend treatment of the undescended testis when detected if the child is older than six months. However, it must be remembered that there are no data to indicate benefit of early treatment. Before treatment is initiated, the physician must be careful to rule out a retractile testis. Also, the possibility of ascent of the testis which may occur during midchildhood, the age of physiologically normal hypogonadotropism, must be remembered. The testis which was previously normally descended but resides much of the time during midchildhood years within the inguinal canal may be a variant of normal and not require therapy. The approach to the patient with cryptorchidism must involve a careful history and repeated examinations looking for a cause and for accurate position of the testis. If the testis can be moved from the nonscrotal position into the scrotum, the potential function of the testis should not be harmed.(ABSTRACT TRUNCATED AT 400 WORDS)