Infertility is reduced or diminished fertility. The most common cause of infertility in males is a low sperm cell count. If the sperm cell count drops to below 20 million sperm cells per milliliter, the male is usually sterile.

Decreased sperm cell count can occur because of damage to the testes as a result of trauma, radiation, cryptorchidism, or infections such as mumps. Reduced sperm cell counts can result from inadequate secretion of luteinizing hormone and follicle-stimulating hormone, which can be caused by hypothyroidism, trauma to the hypothalamus, infarctions of the hypothalamus or anterior pituitary gland, and tumors. Decreased testosterone secretion will also reduce the sperm cell count.

Fertility is reduced if the sperm cell count is normal but sperm cell structure is abnormal, such as chromosomal abnormalities caused by genetic factors. Reduced sperm cell motility also results in infertility. A major cause of reduced sperm cell motility is antisperm antibodies produced by the immune system, which bind to sperm cells.

Fertility can sometimes be achieved by collecting several ejaculations, concentrating the sperm cells, and inserting the sperm cells into the females reproductive tract, a process called artificial insemination.