If a man's testicles are too warm, sperm may not grow correctly.

This can happen if a man wears underwear that is too tight. Or it may happen in men with certain jobs, such as long-distance truck driving.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.