A four-year-old maele/neutered mixed-breed dog was prsented for severe, multifocal, proliferative gingivitis. Histopathologic examination of incisional biopsies supported a diagnosis of Wegener's granulomatosis, an autoimmune vasculitis previously unreported in the veterinary literature. Diagnostic investigations for infectious, neoplastic, or other inflammatory conditions all provided negative results. Management with a combination of immunosuppressive agents resulted in complete resolution and remission of the presenting lesions and associated clinical signs.