Infertility in men is most often caused by:

  • problems making sperm -- producing too few sperm or none at all
  • problems with the sperm's ability to reach the egg and fertilize it -- abnormal sperm shape or structure prevent it from moving correctly

Sometimes a man is born with the problems that affect his sperm. Other times problems start later in life due to illness or injury. For example, cystic fibrosis often causes infertility in men.

Source: The National Women's Health Information Center