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INTRODUCTION and OBJECTIVE. Idiopathic inflammatory myopathies are very rare in infancy. We present five cases of polymyositis in children in which the clinical variability and difficulty in diagnosis that occurs with this disorder are clearly seen, and analyze their response to steroid treatment. CLINICAL CASES. We can distinguish two groups of patients: the first is formed of the case of a two and a half month old baby with generalized hypotonia; and the second includes the other four cases, children of between 2 and 8 years old with weakness, which was mainly proximal. One of the second group later developed juvenile chronic arthritis. In all cases there was a raised CPK and a myopathic EMG, with or without spontaneous muscle activity. Muscle biopsy showed inflammatory myopathy. The first group corresponds to so-called infantile polymyositis which is characterized by generalized hypotonia. The second group includes older children in whom the clinical features of the disorder are similar to those in adults. The association of other autoimmune diseases with infantile polymyositis is exceptional. All cases show more or less improvement with corticosteroids.