Infertility in men is most often caused by:

• A problem called varicocele. This happens when the veins on a man’s testicle(s) are too large. This heats the testicles. The heat can affect the number or shape of the sperm.
• Other factors that cause a man to make too few sperm or none at all.
• Movement of the sperm. This may be caused by the shape of the sperm. Sometimes injuries or other damage to the reproductive system block the sperm.

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.