Article abstract A 68-year-old woman had asymmetrical but bilateral enlargement of the temporalis, masseter, and pterygoid muscles that had begun at the age of 30 and was for the most part progressive. The disorder has been confined to the muscles of mastication; during 15 years of observation no muscles elsewhere have been affected. Histopathologic features are those of a mild myopathic illness, termed branchial myopathy because of the embryologic origin of the muscles concerned. The etiology of this benign disorder is not known, if indeed there is a solitary cause underlying all cases. Surgical treatment may be prompted by mechanical or cosmetic factors.