Harlequin sign associated with Horner syndrome secondary to an 11 cm left upper lung lobe adenocarcinoma

Harlequin sign presents as unilateral flushing and sweating of the face and may occur with Horner syndrome, a condition that involves cervical sympathetic and ocular dysfunction on the nonflushed side.1 The neuronal deficits resulting in Harlequin sign can be explained by a wide range of etiologies, making diagnosis a challenge. We present a rare case of Harlequin sign concomitant with Horner syndrome associated with underlying malignancy.