The Lambert-Eaton myasthenic syndrome (LEMS) is a rare condition in which weakness results from a presynaptic abnormality of acetylcholine release at the neuromuscular junction. It was first described as a paraneoplastic syndrome in patients with lung cancer but we now know about half of the patients with LEMS do not have cancer. The diagnosis is made on the basis of the clinical findings and characteristic electromyographic patterns. Recent evidence indicates that LEMS results from an autoimmune attack directed against the voltage-gated calcium channels on the presynaptic motor nerve terminal. In patients with LEMS who have cancer, effective treatment of the underlying tumor frequently produces marked improvement of weakness as well. Otherwise, treatment involves the use of agents that improve neuromuscular transmission by increasing the release of neurotransmitter, and immunosuppression.