A 70-year-old woman was referred to us for evaluation of a presumed left-sided neck mass. The referral was based on the visual appearance of subtle neck asymmetry. The patient reported no ENT symptoms. Clinical examination and ultrasonography identified no pathology, but computed tomography revealed that the asymmetry was present because the patient's right sternocleidomastoid muscle (SCM) was absent. The SCM on the left side was normal. Congenital absence of an SCM is uncommon, and to the best of our knowledge, an initial presentation in an adult has not been previously reported in the literature.