AbstractForty patients with polymyositis or dermatomyositis underwent detailed electromyographic evaluation. The paraspinal muscles of all patients were examined, as were several extremity muscles. The distribution of fibrillation potentials (FPs) in different muscles is discussed. FPs were most frequent in paraspinal muscles. We conclude that, for any patient suspected of having a myopathy, electromyographic examination should include the paraspinal muscles.