Objective:Rare disease Background:Immunoglobulin G4-related disease (IgG4-RD) is a rare autoimmune disease that can affect multiple organs and manifest itself as a mass at any region of the body. Due to its several differential diagnoses, investigation and treatment are still challenging. Therefore, imaging, serology, and histopathology are required to confirm the diagnosis. The involvement of the temporal bone is an uncommon presentation, often mistaken for malignancy, with vague symptoms. Therefore, we present a 22-year-old Brazilian man, diagnosed with IgG4-related disease, manifesting with unilateral mastoiditis, sensorineural hearing loss, cerebral venous sinus thrombosis, and a mass in the left temporal bone. Case Report:A 22-year-old Brazilian male patient first presented with coughing and precordialgia. Chest scans showed pleural effusion and diffuse areas of ground-glass opacity. A year later, the patient developed severe headache, along with aural fullness, facial pressure, and otorrhea. Imaging detected cerebral thrombosis with failure in the filling of the transverse and left sigmoid sinuses and pachymeningeal thickening in the right cerebral hemisphere, with contrast enhancement. Pure tone audiometry showed thresholds consistent with severe sensorineural hearing loss in the left ear. The patient underwent mastoidectomy with removal of large amounts of inflammatory tissue that were sent to histopathological analysis with compatible signs of IgG4-RD. Corticosteroids and rituximab completed the treatment. Conclusions:Early recognition and appropriate treatment of IgG4-RD are imperative to avoid complications and serious irreversible organ damage. This report has presented an atypical case of IgG4-RD of the left temporal bone that was diagnosed and managed according to current guidelines.