The idiopathic inflammatory myopathies (IIMs) comprise polymyositis, myositis overlapping with another connective tissue disease, dermatomyositis (DM) and inclusion-body myositis (IBM). IIMs are characterized by the presence of proximal muscle weakness, increased levels of muscle-specific enzymes, specific electromyographic abnormalities, and the presence of inflammatory cell infiltrates in skeletal muscle. Clinical, serological and histological criteria can be used to define individual IIM subtypes. In the first of this two-part review series, we examine the evidence for the existence of cancer-associated myositis (CAM), and in part 2, we discuss recent discoveries that provide insight into identification of patients with DM, who may be most at risk of developing CAM.