We report a Japanese infant with Horner syndrome whose clinical examination and 
testing suggested the location of the causative lesion. A 4-year-old Japanese 
girl had an acquired right ocular ptosis and unequal pupils presenting shortly 
after birth. She also exhibited left hemifacial flushing and loss of sweating on 
the contralateral side (harlequin sign). Physical examination demonstrated 2.0 
mm of ptosis of the right upper lid with normal elevator function. The diameters 
of the pupils were 4 mm on the left and 2.5 mm on the right. No sweating was 
induced in the right frontal region at 40 degrees C for 15 minutes of sweat 
challenge test. Otherwise, no abnormalities were found by the neurophysiologic 
examinations or magnetic resonance imaging of the brain. Based on the clinical 
examination, we speculated that the responsible lesion might be in the 
preganglionic areas. Harlequin sign was informative for making the diagnosis of 
Horner syndrome.