The diagnosis of Horner's syndrome (HS) can be difficult, as patients rarely present with the classic triad of ptosis, miosis, and anhydrosis. Frequently, there are no associated symptoms to help determine or localise the underlying pathology. The onset of anisocoria may also be uncertain, with many cases referred after incidental discovery on routine optometric assessment. Although the textbooks discuss the use of cocaine, apraclonidine, and hydroxyamphetamine to diagnose and localise HS, in addition to reported false positive and negative results, these pharmacological agents are rarely available during acute assessment or in general ophthalmic departments. Typically, a week is required between using cocaine or apraclonidine for diagnosis and localisation of HS with hydroxyamphetamine, leaving the clinician with the decision of which investigations to request and with what urgency. Modern imaging modalities have advanced significantly and become more readily available since many of the established management algorithms were written. We thus propose a practical and safe combined clinical and radiological diagnostic protocol for HS that can be applied in most clinical settings.