The aim of this study was to determine if ultrasound could successfully characterize axillary web syndrome (AWS) and clarify the pathophysiologic basis of AWS as a vascular or lymphatic abnormality, or an abnormal tissue structure. This prospective study evaluated women who developed AWS following breast cancer surgery. Using an 18 MHz ultrasound transducer, images were taken of the AWS cord and compared to mirror images on the contralateral side. A blinded radiologist assessed the ultrasound characteristics of and structural changes in the skin and subcutaneous tissue and formulated an opinion as to the side in which AWS was located. Seventeen subjects participated in the study. No structure or abnormality consistent with AWS could be identified by ultrasound. There were no statistical differences between the ipsilateral and contralateral side in skin thickness; subcutaneous reflector thickness, number or disorganization; or subcutaneous tissue echodensity (p>0.05). The radiologist correctly identified the side with AWS in 12 of 17 subjects (=0.41). A distinct ultrasonographic structure or abnormality could not be identified in subjects with AWS using 18 MHz ultrasound. The inability to identify a specific structure excludes the possibility that AWS is associated with vein thrombosis or a fascial abnormality, and supports the theory that AWS may be pathology that is not visible with 18 MHz ultrasound, such as microlymphatic stasis or binding of fibrin or other proteins in the interstitial space.