PURPOSE: To establish imaging criteria for pyomyositis (PM). MATERIALS AND METHODS: Twenty-seven computed tomographic (CT) scans and 11 magnetic resonance (MR) images obtained in 32 patients with PM were reviewed. Images in 10 patients with PM and 16 with soft-tissue masses were evaluated blindly. RESULTS: At CT, all 27 patients had muscle enlargement with heterogeneous attenuation; 26 patients had a focal fluid collection, with rim enhancement in all 18 patients who underwent contrast material-enhanced CT. Twenty-four patients had cellulitis. At MR imaging, all 11 patients had both a subtle increase in signal intensity in the affected muscle(s) on T1-weighted images and cellulitis. Nine patients had a focal fluid collection, which had high signal intensity and a hypointense rim on T2-weighted images. In six patients, a rim of increased signal intensity was seen around the collection on T1-weighted images. Six gadolinium-enhanced examinations demonstrated rim enhancement. Eight patients had fluid in the distal joint. All patients with PM were correctly identified when evaluated with the control subjects; however, there were four false-positive results. CONCLUSION: CT and MR imaging can help characterize changes that are suspicious for PM.