Fibrous dysplasia causing thoracic outlet syndrome is rare. A 41-year-old woman presented with neurogenic thoracic outlet syndrome with imaging that demonstrated a large tumor of her proximal left first rib. Transaxillary excision was unsuccessful due to involvement of the subclavian vasculature and brachial plexus. Subsequent posterolateral thoracotomy and resection of her first rib revealed fibrous dysplasia. Thoracotomy should be considered in these cases for optimal vascular control and identification of thoracic outlet anatomy.