Undescended testicle (aka cryptorchidism, from Greek "crypto" meaning "hidden", and "orchis" meaning "testicle" is the congenital absence of testes (one or both) from the scrotum.

Note that a testis in early infancy can occasionally "reascend" into the inguinal canal. A testis which can readily move between the scrotum and canal is referred to as retractile.

A testis absent from the normal scrotal position can be found:

• Along the path of descent from the high in the posterior (retroperitoneal) abdomen, just below the kidney, to the inguinal ring
• In the inguinal canal
• Ectopically, i.e. to have wandered from its path, usually outside the inguinal canal and sometimes even under the skin of the thigh, the perineum, the opposite scrotum, or the femoral canal
• Undeveloped (hypoplastic) or severely abnormal (dysgenetic)
• To have vanished (anorchia)

• Monorchism, the condition of having only 1 testicle

• Usually self limiting
• To reduce risk, undescended testes can be brought into scrotum in infancy using surgery, known as orchiopexy

Associated with:

• Reduced fertility
• Increased risk of testicular germ cell tumors
• Psychological problems when the boy is grown

Increased risk of:

• Testicular torsion (and subsequent infarction)
• Inguinal hernias

• 80% of undescended testes descend by the 1st year of life, with the majority within 3 months, making the true incidence of non-self limiting undescended testes 1%

• Most common birth defect of the male genetalia
• Can develop later in life, often as late as young adulthood
• 3% of full term, and 30% of premature infants are born with at least 1 undescended testis
• 66% of cases are unilateral, 33% involve both testes
• In 90% of cases, the undescended testis can be felt in the inguinal canal; only a minority are in the abdomen or nonexistent (truly hidden)