We describe MR findings in a case of idiopathic eosinophilic myositis prese
nting as painful swelling of the upper arm. Imaging revealed diffusely enla
rged and edematous triceps muscle with an associated ill-defined heterogene
ous mass in its midportion. Prominent edema was noted in adjacent fascia. T
he follow-up MR image showed complete resolution, though no specific treatm
ent was given.