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.