A true costoclavicular mass syndrome associated with a brachial plexopathy is rare. We report the occurrence of a severe brachial plexopathy as a late complication of a displaced midclavicular fracture. An exuberant callus associated with the clavicular fracture acted as a mass lesion to compress the brachial plexus within the costoclavicular space (i.e., between the clavicle and the first rib). The clinical features and the electrodiagnostic findings in this patient were crucial in suggesting the diagnosis, which was subsequently confirmed by radiographic studies and surgical exploration. Surgical excision of the hyperabundant callus and freeing of the entrapped brachial plexus resulted in marked improvement of the patient's neurological symptoms. Recognition of this uncommon complication of a clavicular fracture is important for the timely diagnosis of this treatable problem.