Of 450 cases of Horner's syndrome examined by pupillography. 13% were caused by tumor but less than 3% were undetected malignant tumors. The largest group of lesions (40%) had an undetermined cause, presumably related to vascular disease. Although the hydroxyamphetamine test usually can differentiate postganglionic from preganglionic lesions, its clinical value and accuracy for this purpose has been overemphasized in previous reports. Of the 13 undetected malignant neoplasms, nine (69%) manifested as Pancoast's syndrome with arm pain, a reliable sign to identify patients with Horner's syndrome caused by undetected malignancy.