Lambert-Eaton syndrome (also called Eaton-Lambert syndrome) is a very rare condition in which a person's immune system attacks nerve tissues it mistakenly identifies as invading organisms. As a result, electrical signals do not travel between nerves and muscles as they should, which typically causes weakness in the upper arm and thigh muscles.

Other symptoms may include a dry mouth, muscle pain, vision problems, and droopy eyelids (ptosis). Men with Lambert-Eaton syndrome may also be unable to achieve or maintain an erection.

Certain forms of cancer can sometimes trigger the development of Lambert-Eaton syndrome.