Myofibromas are mesenchymal tumors that most often present in infancy. There are sporadic cases in adults throughout the literature. Myofibroma will demonstrate a variable appearance on CT and ultrasound, but tends to have a fairly consistent appearance on MR imaging. This report describes the MR and sonographic findings as well as the pathologic correlation of a myofibroma in the arm of a middle-aged woman. The patient was well, without evidence of recurrence 6 months following resection.