THE polymyositis and dermatomyositis complex encompasses a heterogeneous group of acquired muscle diseases called inflammatory myopathies because muscle weakness and inflammatory infiltrates within the skeletal muscles are the principal clinical and histologic findings.^{1 2 3 4 5 6 7 8 9 10 11 12} Traditionally, polymyositis and dermatomyositis have been viewed as pathogenetically similar and part of the spectrum of "idiopathic" inflammatory myopathies,^{9 , 10 , 13 , 14} despite the variability in their clinical and laboratory characteristics, prognosis, and response to therapy. With the evolution over the past 10 years of rather well defined clinical, demographic, histologic, and immunopathological criteria and the identification of inclusion-body myositis as a distinct type of polymyositis,¹⁵ the inflammatory myopathies now . . .