We studied hydroxyamphetamine hydrobromide 1% (Paredrine) mydriasis in 54 patients with Horner's syndrome to determine its effectiveness in distinguishing preganglionic lesions from postganglionic lesions. The difference in pupillary dilation between the unaffected and affected sides was used as a measure of the hydroxyamphetamine effect. We found that patients who had clinical evidence of damage to the postganglionic neuron of the oculosympathetic pathway had less pupillary dilation on the affected side. In contrast, almost all patients judged to have clinical evidence of preganglionic lesions dilated more on the affected side. We determined the probability that a given difference in pupillary dilation between the involved and uninvolved side is the result of a postganglionic lesion.