The term "Raeder's syndrome," which now seems to mean any painful postganglionic Horner's syndrome, has been used in the past to describe patients with a wide variety of underlying pathology, including such serious lesions as middle cranial fossa neoplasms and such benign conditions as unilateral vascular headache syndromes. The purpose of this review which is based on the literature and some recent experience with 41 cases of Raeder's syndrome, is to help clarify this syndrome and to aid the clinician in its evaluation and treatment. Patients with Raeder's syndrome have been divided into three major groups. In the first group, the painful postganglionic Horner's syndrome is associated with multiple parasellar cranial nerve involvement and these patients require full neuroradiological investigation to uncover such lesions as local or metastatic tumors within the middle cranial fossa. The second and third groups do not have the multiple cranial nerve damage and their prognoses are benign. The characteristics, clinical investigation and medical therapy of each of these two benign groups are outlined and discussed. Extensive neuroradiological investigation is not recommended for patients in the second or third groups. Common to all three groups of Raeder's syndrome is the association of unilateral headache with the interruption of the postganglionic oculosympathetic fibers along the course of the internal carotid artery.