When trying to establish the likely anatomical site (preganglionic or 
postganglionic) of a lesion causing congenital Horner's syndrome, the 
distribution of facial flushing (the "harlequin" sign), may be seen. In babies 
and young children, facial flushing is a relatively simple clinical sign to 
demonstrate, compared with facial sweating. In unilateral facial flushing the 
areas that do not flush are almost always identical to the anhidrotic areas. 
However, neither facial flushing nor testing the pupil reactions with pholedrine 
or hydroxyamphetamine can be relied on to predict the probable site of any 
lesion causing congenital Horner's syndrome. Two patients with congenital 
Horner's syndrome are presented which demonstrated the "harlequin" sign and in 
whom clinical examination and pharmacological testing gave conflicting evidence 
for localisation of the site of the causative lesion. The presentation of 
congenital Horner's syndrome should be investigated and include MRI or CT to 
exclude a serious underlying cause.