BACKGROUND: Musculoskeletal impairments of the upper extremity following mastectomy are well documented, although the cause of these impairments is not always known. CASE PRESENTATION: Asymmetry of the shoulder girdle musculature was found during anterior chest wall dissection of a female cadaver with a history of a left mastectomy at the age of 45 years (age at death: 96 years). The left pectoralis major and minor muscles were markedly smaller than the contralateral pectoralis muscles, whereas the converse was apparent for the subclavius muscles. OUTCOME AND FOLLOW-UP: In situ measurements of the depth of the bellies of the subclavius muscles demonstrated that the left subclavius depth was nearly twice that of the right on the initial donor. Measurements of the length and wet weight of the muscles confirmed both the increased size and mass of the left subclavius. Further research on an additional 21 pairs of subclavius included 2 additional donors with a mastectomy. When pooled with the original donor's data, the combined 3 subclavius pairs following mastectomy showed a statistical difference in mass (a1 = .05, P = .0354) and width (a1 = .05, P = .0014). DISCUSSION: The pattern of atrophy of the pectoralis muscles in our primary case suggests that the medial pectoral nerve was damaged during surgery or during postoperative treatment. How this led to the apparent hypertrophy of the left subclavius muscle is unclear. It is possible that because of the atrophy and associated weakness of the left pectoralis muscles, the left subclavius muscle took on a greater role in shoulder girdle stabilization, increasing in size and mass over time. JOSPT Cases 2023;3(2):127–133. Epub: 13 March 2023. doi:10.2519/josptcases.2023.11561