The idiopathic inflammatory myopathies (IIM) are a heterogeneous group of systemic diseases that include the familiar disease entities of dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM). A subset of patients has unique autoantibodies which are specific for IIM (myositis specific autoantibodies; MSAs). We studied the clinical and serological characteristics of IIM in 125 Dutch patients. Sera were analysed by immunoblotting, enzyme-linked immunosorbent assay, and immunoprecipitation. The most frequently encountered MSA was the anti-Jo-1 autoantibody (20%), followed by anti-tRNAHis (6%), anti-Mi-2 (6%), and anti-SRP (4%). The presence of certain MSAs was clearly associated with specific clinical characteristics. Anti-Jo-1 and anti-tRNAHis were associated with the anti-synthetase syndrome, anti-SRP with PM with severe myalgia and arthralgia and a moderate response to immunosuppressive treatment. A novel finding was the presence of anti-Mi-2, not only in DM, but also in PM. MSAs were frequently present in DM/PM sera, but were hardly ever detected in the sera of IBM patients. The few IBM patients with MSAs demonstrated a significant response to immunosuppressive treatment. It can be concluded that MSAs define specific clinical syndromes within the spectrum of IIM and that they can assist in the differential diagnosis and treatment plan of these enigmatic disorders by virtually excluding IBM by their presence, and by potentially identifying a subgroup of steroid-responsive IBM patients.