A patient with Horner syndrome from internal carotid artery dissection initially had a false negative hydroxyamphetamine test. Two months later, the ophthalmic signs had disappeared but the hydroxyamphetamine test was positive. This case illustrates that hydroxyamphetamine testing may be falsely negative in acute Horner syndrome because norepinephrine stores in oculosympathetic postganglionic terminals have not yet been depleted. However, the hydroxyamphetamine test may be positive even after the ophthalmic signs of Horner syndrome have disappeared.