UNEQUAL PUPILS: Or anisocoria is frequent and can be physiological, benign or life threatening. VARIATIONS IN THE PUPIL: Constriction depends on the parasympathetic system and dilatation on the sympathetic system. IN PARASYMPATHETIC DISORDERS: Peripheral or central neurological causes can be distinguished, among which acute cerebral lesions with compression of the trunk are the most dramatic. Differential diagnosis can be made by studying the local causes, which may represent ophthalmologic emergencies (acute glaucoma or lesions of the retina). AMONG THE SYMPATHETIC DISORDERS: Claude-Bernard-Horner's syndrome is the most classical example. Other particular cases exist, such as Adie's pupil, affecting young women (1/20,000), associating mydriasis and occasionally tendinous areflexia. Diagnosis is generally confirmed by the ophthalmologist, following a normal or evocative of the disease neurological examination.Rapid but careful clinical examination permits orientation of the diagnosis and appropriate treatment.