A comprehensive study of 194 Chinese patients with myasthenia gravis in Hong Kong has shown distinct differences from the patterns of disease seen in Caucasians. Restricted ocular myasthenia is the predominant disease type in patients presenting in the first two decades of life and is associated with absence or low titres of acetylcholine receptor antibody. Predisposition to this type of disease is strongly associated with HLA-DRw9. Generalized myasthenia gravis occurs predominantly in patients presenting after the age of 20 years and is accompanied by high titres of acetylcholine receptor antibody but is less strongly associated with HLA-DRw9. It is suggested that myasthenia gravis occurring within the first two decades of life and characterized by affected ocular muscles and absence or low titre of acetylcholine receptor antibody is a genetically determined variant of the disease which occurs commonly in Orientals.