[3] A testis absent from the normal scrotal position may be: About two-thirds of cases without other abnormalities are unilateral; most of the other third involve both testes.

Undescended testes are associated with reduced fertility, increased risk of testicular germ-cell tumors, and psychological problems when fully-grown.

[4] Although cryptorchidism nearly always refers to congenital absence or maldescent, a testis observed in the scrotum in early infancy can occasionally "reascend" (move back up) into the inguinal canal.

The basis for the universal recommendation for early surgery is research showing degeneration of spermatogenic tissue and reduced spermatogonia counts after the second year of life in undescended testes.

[7] In most full-term infant boys with cryptorchidism but no other genital abnormalities, a cause cannot be found, making this a common, sporadic, unexplained (idiopathic) birth defect.

A combination of genetics, maternal health, and other environmental factors may disrupt the hormones and physical changes that influence the development of the testicles.

In 2008, a study was published that investigated the possible relationship between cryptorchidism and prenatal exposure to a chemical called phthalate (DEHP), which is used in the manufacture of plastics.

The researchers found a significant association between higher levels of DEHP metabolites in pregnant mothers and several sex-related changes, including incomplete descent of the testes in their sons.

[12] A 2010 study examined the prevalence of congenital cryptorchidism among offspring whose mothers had taken mild analgesics, primarily over-the-counter pain medications including ibuprofen (e.g. Advil) and paracetamol (acetaminophen).

[13] Combining the results from a survey of pregnant women prior to their due date in correlation with the health of their children and an ex vivo rat model, the study found that pregnant women who had been exposed to mild analgesics had a higher prevalence of baby boys born with congenital cryptorchidism.

[13] New insight into the testicular descent mechanism has been hypothesized by the concept of a male programming window derived from animal studies.

[15] Animal studies have found that androgen antagonists during early fetal development cause elevated rates of hypospadias and cryptorchidism, however, in humans these traits rarely occur together.

[16] The testes begin as an immigration of primordial germ cells into testicular cords along the gonadal ridge in the abdomen of the early embryo.

The first phase, movement across the abdomen to the entrance of the inguinal canal, appears controlled (or at least greatly influenced) by anti-Müllerian hormone (AMH).

In rodents, androgens induce the genitofemoral nerve to release calcitonin gene-related peptide, which produces rhythmic contractions of the gubernaculum, a ligament which connects the testis to the scrotum, but a similar mechanism has not been demonstrated in humans.

Some circumstantial evidence suggests tight underwear and other practices that raise the testicular temperature for prolonged periods can be associated with lower sperm counts.

Subtle or transient hormone deficiencies or other factors that lead to a lack of descent also may impair the development of spermatogenic tissue.

The inhibition of spermatogenesis by ordinary intra-abdominal temperature is so potent that continual suspension of normal testes tightly against the inguinal ring at the top of the scrotum by means of special "suspensory briefs" has been researched as a method of male contraception, and was referred to as "artificial cryptorchidism" by one report.

In normal males, as the cremaster muscle relaxes or contracts, the testis moves lower or higher ("retracts") in the scrotum.

Scrotal ultrasound or magnetic resonance imaging performed and interpreted by a radiologist can often locate the testes while confirming absence of a uterus.

[citation needed] A karyotype can confirm or exclude forms of dysgenetic primary hypogonadism, such as Klinefelter syndrome or mixed gonadal dysgenesis.

In the even smaller minority of cryptorchid infants who have other obvious birth defects of the genitalia, further testing is crucial and has a high likelihood of detecting an intersex condition or other anatomic anomalies.

The presence of a uterus by pelvic ultrasound suggests either persistent Müllerian duct syndrome (AMH deficiency or insensitivity) or a severely virilized genetic female with congenital adrenal hyperplasia.

Nevertheless, despite the potential advantages of a trial of hormonal therapy, many surgeons do not consider the success rates high enough to be worth the trouble, since the surgery itself is usually simple and uncomplicated.

In cases where the testes are identified preoperatively in the inguinal canal, orchiopexy is often performed as an outpatient and has a very low complication rate.

The testis with accompanying cord structure and blood supply is exposed, partially separated from the surrounding tissues ("mobilized"), and brought into the scrotum.

In the worst case, the testis must be "autotransplanted" into the scrotum, with all connecting blood vessels cut and reconnected (anastomosed).

When the testis is in the abdomen, the first stage of surgery is exploration to locate it, assess its viability, and determine the safest way to maintain or establish the blood supply.

Castration of the undescended teste(s) should be considered for cryptorchid dogs due to the high rate of testicular cancer, especially Sertoli cell tumors.

[26] Urine spraying is one indication that a cat with no observable testicles may not be neutered; other signs are the presence of enlarged jowls, thickened facial and neck skin, and spines on the penis (which usually regress within six weeks after castration).