Aim: To characterise the clinical and pathological features of 24 patients with biopsy proven Idiopathic Orbital Inflammatory Syndrome (IOIS). Methods: Retrospective case series. Results: The study included 14 men and 10 female patients, ranging in age from 14 to 75 years. The presenting symptoms and signs included pain (14/24), swelling/mass (19/24), diplopia (9/24), proptosis (15/24), extraocular muscle restriction (13/24), decreased vision (5/24) and ptosis (4/24). Histopathology was varied and included classical orbital pseudotumour (9/24), sclerosing orbital pseudotumour (13/24), vasculitic orbital pseudotumour (1/24) and granulomatous orbital pseudotumour (1/24). Treatments included oral steroids (19/24), intravenous steroids (1/24), methotrexate, azathioprine, mycophenalate and ciclosporin. Forty-two per cent of the patients had recurring episodes during the follow-up period, with 29% of patients requiring two or more treatment regimens to maintain remission. Two-thirds of patients (16/24) had complete resolution of their symptoms and signs. There was no correlation between the histopathological subtype, relapse rate or symptoms and resolution of signs. Conclusion: Idiopathic Orbital Inflammatory Syndrome has variable clinical and pathological features. Although, in some patients, symptoms and signs resolve spontaneously, most require treatment with oral steroids and additional immunosuppressant drugs or radiotherapy. The clinical and pathological features do not correlate with treatment outcomes.