Editor, In August 2013, a 17-year-old girl presented with progression of severe headaches accompanied with double vision for 2 weeks. Ophthalmologic examination showed bilateral papilledema and right-sided abducens paresis. Whilst extensive neurological screening (CT and MRI) showed no abnormalities, lumbar puncture revealed an increased intracranial pressure (>50 cm H2O, with normal CSF constituency) supporting the diagnosis of idiopathic intracranial hypertension (IIH). A successful ventriculo-peritoneal shunt was placed. Two years later however, the patient returned with a painful and red left eye. Ophthalmologic examination revealed impaired abduction caused by a diffuse swelling of the left medial rectus muscle (Fig. 1A,B). After exclusion of systemic involvement, the patient was diagnosed with non-specific orbital inflammation (NSOI) and successfully treated with oral prednisolone. In November 2013, the identical twin presented with severe pain on eye movement of the left eye. Examination showed a significant proptosis and MRI revealed an enlargement of the left medial rectus muscle, exactly at the same location as her sister (Fig. 1C,D). Without abnormalities in systemic workup, the diagnosis of NSOI was established. Treatment was initiated with 3-day IV methylprednisolone (500 mg) followed by oral prednisolone, with a good response. In contrast to her sister, multiple relapses occurred for which additional therapies were given, including methotrexate, biologics, radiotherapy and IV immunoglobulins with minimal results. Eighteen months after diagnosis, the patient returned with a diffuse headache, decreased visual acuity and papilledema bilaterally. CT and MRI of the cranium showed no abnormalities, but lumbar puncture revealed an increased intracranial pressure (>50 cm H2O, with normal CSF constituency). Similar to her sister, a ventriculo-peritoneal shunt was placed. Until today, oral prednisone is needed for treatment of her NSOI. No other family members were affected by any of the symptoms or diagnoses. NSOI is a benign, non-infectious, inflammatory disease of which the diagnosis is based on exclusion of specific local or systemic causes (Yuen & Rubin 2003). A biopsy should be performed when malignancy is suspected or no effect of treatment is noticed (Bijlsma et al. 2012). Relapses after treatment occur frequently and NSOI can remain refractory (Yuen & Rubin 2003). IIH is a disorder with signs and symptoms of raised intracranial pressure, with no established pathogenesis (Markey et al. 2016). The predominant clinical feature of IIH is headache, although diagnosis is made through a combination of increased intracranial pressure with either papilledema or a 6th nerve palsy (Markey et al. 2016). Treatment consists of lifestyle, pharmaceutical and/or surgical interventions. Early and adequate treatment is needed to prevent permanent visual loss. IIH and NSOI can have overlapping clinical features complicating individual diagnosis. A clinical tool for differentiation is the distinction between paresis and mechanical limitation of eye movement. Within literature, no cases of coexisting IIH and NSOI have previously been reported, and the conditions were previously considered unrelated. Several reports in families suggest possible (epi)genetic factors that confer risk for developing these conditions (Yuen & Rubin 2003, Jacob et al. 2007, Markey et al. 2016). However, no genetic loci have yet been linked to either conditions. The co-occurrence of IIH and NSOI reported here in twins, with presentation in reversed order within a time period of 2 years, suggests possible shared disease mechanisms. An association between IIH and NSOI can be used in future research for both conditions.