Patients with thoracic outlet syndrome can be treated with osteopathic manipulative treatment (OMT) to alleviate dysfunction and restriction of the pectoralis minor muscle (PMM) and the resulting compression of the brachial plexus. Neuromuscular ultrasonography (US) can demonstrate abnormalities in the thoracic outlet that are amenable to OMT and can be used to monitor intervention. The present report identifies PMM deformation and brachial plexus compression in a 32-year-old woman with thoracic outlet syndrome who was treated successfully with OMT. Neuromuscular US results were used to measure the degree of PMM deformation with the pectoral bowing ratio and confirm the diagnosis. Osteopathic manipulative treatment was applied and monitored using neuromuscular US to confirm that the operator's manipulating hand had direct contact with the PMM. Symptoms abated immediately after treatment. Results of a second neuromuscular US examination showed that the pectoral bowing ratio decreased into the normal range and thus confirmed that PMM deformation had resolved.