ptic perineuritis (OPN) and acute optic neuritis (ON) are clinically similar disorders. Sometimes, ocular motility disturbances, ptosis, and chemosis can accompany OPN. While ON is common in multiple sclerosis, OPN can be related to vascular, toxic, infectious, metabolic, neoplastic and inflammatory etiologic factors. Unlike demyelinating ON, central vision is spared in OPN, and it responds well to corticosteroids but recurrence is more likely to occur. Differential diagnosis of these disorders is usually dependent on radiologic features. 1,2 Fat-suppressed, contrasted magnetic resonance imaging (MRI) sequences can aid in recognizing the distinction between the two disorders. 3 The differential diagnosis is very important because the prognosis and the response to treatment are different. 1,2 In this article, we report three rare cases with clinical, ophthalmologic and radiologic findings. CASE REPORTS CASE 1 A seventy-five-year-old man was referred to our outpatient clinic complaining of the sudden onset of blurring, redness, and pain in the left eye for two days. Neurologic examination showed restricted abduction of the left eye in a conjugate horizontal gaze with a mildly dilated left pupil. Edema of Optic Perineuritis: How Does it Differ from Optic Neuritis? A AB BS S T TR RA AC CT T Sometimes, a diagnostic error between optic neuritis (ON) and optic perineuritis (OPN) can occur. The differential diagnosis of these two entities is very important because the prognosis and response to treatment are different from each other. We describe here three unusual OPN cases that presented with the symptom of sudden onset blurred vision and are associated with different etiologies (Familial Mediterranean fever, Lyme disease and possible Behcet's disease). Ophthalmological examination and optical coherence tomography can not clearly discriminate ON from OPN, however, magnetic resonance imaging of the optic nerve can help form a diagnosis through the typical appearance of optic nerve enhancement and myeline sheath detachment. These cases are discussed in detail in this article.