[corrected] To determine the relative importance of the different forms of anisocoria in a General Hospital.A prospective, longitudinal study was conducted including all patients referred for this reason to the Neuro-Ophthalmology Unit of the Henares University Hospital, Madrid (Spain), from November 2008 to October 2011. The differences in pupil diameter were studied under high and low luminosity. The patients were given a full ophthalmological examination, as well as performing the apraclonidine, cocaine, pilocarpine 0.125% and pilocarpine 2% tests, if they were considered necessary.Thirty-two cases of anisocoria were referred during the three years of the study. No relationship was found with age or gender. The diagnostic results were: Adie's pupil, 4 cases; Horner syndrome, 5 cases; benign episodic unilateral mydriasis, 3 cases; local causes, 4 cases; physiological anisocoria, 5 cases. Despite a full clinical history and examination, the cause of the anisocoria could not be determined in 11 cases. In 4 of these cases, the patient suffered from migraines and in another 4 psychotropic drugs were taken. Both risk factors were present in 3 cases. In one case the anisocoria was the initial clue that led to the diagnosis of a cervical paraganglioma.Anisocoria is a clinical sign that does not usually signify a serious disease. With our protocols, a high number of anisocoria cases are still of unknown origin. Migraines and psychotropic drugs could be linked to these forms of anisocoria.