A 67-year-old man presented with a 3-week history of diplopia and left ocular pain. Examination showed limitation of abduction of the left eye. Computed tomographic scan demonstrated a soft tissue density mass in the ethmoid sinus (yellow arrow) with extension into the left orbit and infiltration of the medial rectus muscle (Fig. 1A). Incisional biopsy revealed a dense lymphoplasmacytic infiltrate (Fig. 1B), storiform fibrosis, and obliterative phlebitis (yellow arrow, Fig. 1C). Immunohistochemistry was positive for immunoglobulin 4 (IgG4; IgG4+ cells > 30/hpf and IgG4:IgG ratio > 40%) (Fig. 1D; yellow arrow shows IgG4+ plasma cell). Serum IgG4 levels were not measured, and the patient had no history of other autoimmune or IgG4-related systemic disease. The patient was started on oral prednisone, and infliximab was administered later to stop recurrences associated with gradual oral steroid tapering.