The classification of 135 patients with either dermatomyositis (DM) or polymyositis (PM) showed 34 cases of DM and 56 cases of PM in adults, 6 cases of DM associated with cancer, 9 cases of DM in children, 16 cases of localized PM and 14 cases of an overlapping syndrome. Results of biological tests, erythrocyte sedimentation rate and serum enzyme determinations were inconstantly abnormal. The electromyograms were generally of a myogenic type with spontaneous activity in about half the cases. Muscle biopsy usually showed inflammation necrosis and regeneration, sometimes only of moderate severity only. Results were normal in several cases. In 21 patients only the pathognomonic perifascicular atrophy was reported. Proposed classifications are unsatisfactory. Polymyositis may be considered as a syndrome. Among the primary forms the distinction between acute dermatomyositis and subacute or chronic polymyositis is poorly defined and passage from one disorder to another is frequent. Pseudo-myasthenic forms are not entities and pseudo-myopathic types are actually muscular dystrophies. Associations with polymyositis are common and may consist only in the addition of one sign of no clinical significance (PM "plus"). The polymyositis lesion may be part of a syndrome such as Gougerot-Sjögren's or of another connective tissue disease. A system of diagnostic criteria uses numerical ratings of each criterion as a function of its semiologic importance.