Myasthenia gravis is an autoimmune disorder in which neuromuscular transmission is impaired by autoantibodies to the acetylcholine receptor (AChR). There are 3 varieties of generalised myasthenia with differing genetic susceptibilities. There is also a purely ocular form in which the weakness is confined to the extraocular muscles, and a neonatal variety which occurs in 20% of babies born to myasthenic mothers due to transplacental passage of the acetylcholine receptor antibody. Another variety of myasthenia occurs several months after treatment with D-penicillamine.The role of the thymus is suggested by abnormal histology in patients with myasthenia and by the beneficial effects of thymectomy in more than two-thirds of patients. Thymectomy is indicated in most patients unless the symptoms are minimal or the weakness is confined to the extraocular muscles.Most patients require treatment with anticholinesterase drugs to prolong the action of acetylcholine at the muscle end-plate. Overdosage of these drugs can provoke a cholinergic weakness.Remissions can be achieved with corticosteroids in 80% of patients. Immunosuppression with azathioprine is used mainly in patients who do not respond to thymectomy or in those patients who are considered unsuitable for operation. Plasma exchange can cause a rapid but temporary improvement in myasthenia, and has no long term place in its treatment.