We examined the usefulness of magnetic resonance imaging (MRI) in detecting active muscle disease in 40 patients with idiopathic inflammatory myopathies (IIM). Ten patients without evidence of an inflammatory neuromuscular disease were also studied. The fat-suppressive (STIR) image signal intensity correlated with clinical disease activity and, in most cases, with the presence of inflammation on muscle biopsy. Increased STIR signal intensity paralleled disease activity in 3 patients followed serially. MRI provided a detailed anatomic view of the extent of muscle changes in these diseases. Because of inherent limitations of other measures of disease in these disorders, MRI may prove to be a useful complimentary test for assessing disease activity and guiding therapeutic decisions and biopsy in the idiopathic inflammatory myopathies.