The diagnosis of Horner syndrome (HS) using apraclonidine eye drops is an alternative to the use of topical cocaine drops. A number of reports have described the efficacy of apraclonidine testing, but there is some debate over its sensitivity in the acute setting. We describe a patient with HS secondary to carotid dissection who had a positive response to apraclonidine 3 hours after the onset of symptoms. The case is made for a larger study of apraclonidine use to determine its true sensitivity and specificity, identify confounding factors, and redefine the criteria for positive testing.