Asymmetrical facial sweating and flushing has been named the "Harlequin Sign". This is a rare feature, as evident from only 12 cases described up to date. The "Harlequin Sign" represents a local autonomic dysfunction due to a cervical sympathetic deficit located at the pre or postganglionic level on the non-flushing side. We observed slow onset and progression of the "Harlequin Sign" in a 19-year-old man, with preexisting slight miosis on the non-flushing side. The differential diagnosis included other forms of dysautonomia and a secondary origin of this partial Horner's syndrome. Both pupils normally reacted to light, convergence, and pilocarpine eye-drop instillation, but the affected side showed supersensitivity to phenylephrine. Deep tendon reflexes were normal, thus excluding Adie's syndrome. The absence of cholinergic supersensitivity in the iris muscles indicated normal function of the ciliary ganglion and excluded the ocular parasympathetic deficit, also evident for the coexisting Horner's syndrome. The clinical features could be explained by assuming that the lesion was located at the level of postganglionic sympathetic fibers, probably due to trans-synaptic postganglionic neuronal degeneration at the level of the stellate ganglion, thus determining the onset of the hemifacial symptoms.