Rupture of the pectoralis major muscle is rare. The majority of cases have been reported in young, healthy athletes during strenuous activities such as weight-lifting, wind surfing, and football. The injury is thought to result from either a forceful contraction of the muscle applied to an immovable object or a sudden stretching force applied to the contracting muscle. This report describes a case of a 79-year-old man with a history of severe degenerative joint disease of the right hip and a recent left transtibial amputation secondary to a failed arterial bypass procedure. During prosthetic training the patient developed acute pain in the left shoulder and left axilla, which prevented him from further ambulation. Physical examination was consistent with a partial rupture of the pectoralis major. The causative factors of pectoralis major rupture in this patient are reviewed. To our knowledge, this is the first such case reported.