The activity of anti-filamin (F) and anti-vinculin (V) antibodies (Ab) was determined in sera from patients with myasthenia gravis (MG), polymyositis (PM) and Duchenne muscular dystrophy (DMD). Compared with normal controls, activities of both antibodies were significantly (p<0.005) higher in sera from patients with MG and PM, especially with a marked increase in anti-F Ab in ocular myasthenia. In sera from patients with DMD, both activities were slightly higher. Filamin and vinculin are known to be enriched at the periphery of the Z-discs and in the postsynaptic structures of the neuromuscular junction in skeletal muscle. This suggests that these autoantibodies may play some role in the pathogenesis of MG and PM.