The site of autonomic deficit was investigated in 4 patients with loss of thermoregulatory flushing and sweating on one side of the face but no associated miosis (harlequin syndrome). In 2 patients the sudomotor deficit included the hand. Signs of postganglionic sympathetic deficit included pathological lacrimal sweating (2 patients) and supersensitivity to 1% phenylephrine eyedrops (3 patients). One or both pupils constricted excessively to 0.0625% pilocarpine eyedrops (3 patients), consistent with ocular parasympathetic deficit. These findings suggest that pre- or postganglionic cervical sympathetic fibers, and parasympathetic neurons in the ciliary ganglia, are compromised in harlequin syndrome. The finding of cholinergic supersensitivity in the iris muscles of patients with harlequin syndrome indicates a relationship with Holmes-Adie syndrome, Ross's syndrome, and the persistent autonomic deficit occasionally associated with Guillain-Barré syndrome.