FHL1 (Four-and-a-half-LIM domain 1) is identified as a new muscle specific autoantigen. Detection of autoantibodies for FHL1 in a sample obtained from a human subject are useful for diagnosis of autoimmune muscle diseases, including myositis, polymyositis, dermatomyositis, inclusion body myositis, and immune mediated necrotizing myopathy. FHL1 and FHL1 derived peptides are used for the treatment of autoimmune muscle diseases.