Edge-light pupil cycle time has been introduced recently as a simple method for examining parasympathetic pupillary innervation. Since the sympathetic system is not thought to be involved in the light reflex, it was assumed that edge-light pupil cycle time would be unimpaired in lesions of the sympathetic pathways. In 12 patients with unilateral Horner's syndrome of different etiologies, the edge-light pupil cycle time was determined in both eyes. In all cases the edge-light pupil cycle time was significantly prolonged on the abnormal side. Patients with central, preganglionic, or postganglionic lesions showed the differences consistently. These data support the notion that the sympathetic system plays a role in the pupillo-dilating phase of edge-light pupil cycle time, presumably by exerting a tonic mydriatic effect. Since this tonic effect is lost in Horner's syndrome, leading to slowed redilatation, the edge-light pupil cycle time becomes prolonged. Thus, edge-light pupil cycle time may prove to be a valuable diagnostic test for Horner's syndrome.