This is a study to assess the efficacy and safety of 3,4-diaminopyridine (DAP) in patients with Lambert-Eaton myasthenic syndrome (LEMS). LEMS is a rare disorder of neuromuscular transmission that produces weakness of varying severity and autonomic dysfunction. It is frequently associated with small cell lung cancer and results from an autoimmune reaction against the presynaptic nerve terminal. DAP is an orphan drug that has been used in other countries for almost 20 years to treat patients with LEMS and other diseases with abnormal neuromuscular transmission, but which has not been approved for clinical use in this country. Treatments for LEMS include appropriate therapy for any underlying cancer, which frequently produces improvement in weakness as well; cholinesterase inhibitors, which usually produce only limited benefit; guanidine, which can produce severe side-effects; immunosuppression, which is usually only minimally or moderately effective; and plasmapheresis or high-dose immunoglobulin infusions, which produce only temporary improvement and are very expensive. Many patients with LEMS achieve marked improvement in symptoms and function with DAP. This is a blinded, placebo-controlled study to assess efficacy and a chronic open-label phase to assess safety of DAP in patients with LEMS. During the blinded phase of the study, patients are hospitalized on the Clinical Research Unit for 10 days to observe therapeutic responses and potential adverse reactions to the study drug. Patients may also be hospitalized on the CRU when they return for 3 follow-up evaluations during the 6 month open-label phase of the study.