Optic nerve perineuritis targets the optic nerve sheath; it is idiopathic or a manifestation of systemic inflammatory diseases such as myelin oligodendrocyte glycoprotein (MOG) antibody syndrome, sarcoidosis, granulomatosis with polyangiitis, IgG4-related disease, or giant cell arteritis (GCA). 1 Radiographically, there is optic nerve sheath enhancement and, occasionally, orbital fat "streaking." 1 Biopsies have identified dural sheath lymphocytic infiltrate, perineural fibrous tissue, granulomas, or evidence of small-vessel vasculitis. 1 Retroperitoneal fibrosis (RPF) is defined by abdominal organ fibrosis. Usually idiopathic, it may be secondary to medication, radiation, or systemic inflammatory conditions. 2 Pathologically, its hallmark is fibrosis and infiltration of macrophages, B-lymphocytes, and T-lymphocytes. 2 Often considered when older patients present with headache or ischemic optic neuropathy, GCA is the most common largevessel vasculitis. 3 It has been proposed that GCA results from a T-cell-mediated inflammatory cascade, causing vascular intimal hyperplasia. 3 GCA has rarely been reported in association with RPF 4,5 or perineuritis. 6,7 We present a patient initially diagnosed with idiopathic RPF treated with immunosuppression, who subsequently developed optic perineuritis followed by GCA.