Wegener's granulomatosis (WG) is a necrotizing vasculitis that classically involves the upper and lower respiratory tracts and kidneys. Uncommonly, other sites may also be involved. We report on a patient previously diagnosed with and treated for WG who presented with flank pain and on further imaging was found to have a retroperitoneal mass. A surgical specimen of the tissue revealed multiple foci of necrotizing vasculitis. Consideration should be given to the possibility that mass-like lesions in patients with WG may not be tumors since the management and outcome differ from that of an active vasculitis.