THE term "orbital pseudotumour" has been used to describe inflammatory lesions of the orbital tissues of unknown aetiology simulating a neoplasm of the orbit. The term is widely accepted by ophthalmologists, little known to neurosurgeons, and puzzling to pathologists. The term is inadequate and, to a certain extent, even misleading.It is inadequate. because it denotes a completely negative approach to this condition. These lesions present a definite clinical entity. The similarity to an orbital neoplasm is only superficial. The name is also misleading because the condition presents itself quite frequently as a true tumour, though this is an inflammatory tumefaction and not a neoplastic one.An alternative name for this lesion is "nonspecific granuloma of the orbit". This term, which has found some favour among pathologists (Easton and Smith, 1961), is correct if the word granuloma is used in the ancient, macroscopic way, characterizing a tumefaction not caused by a neoplasm. However, the term granuloma is nowadays used in the microscopic sense to designate an inflammatory reaction characterized by the presence of epithelioid cells and giant cells, and such a histological picture, though occasionally present, is the exception, not the rule.In the absence of any better name and because of the impossibility of using an aetiological designation, the widely-accepted term "inflammatory pseudotumour of the orbit" will probably remain in use.Since the original description and classification of Birch-Hirschfeld (1905), the approach to this problem has been somewhat negative; he included cases of exophthalmos which disappeared spontaneously or on oral medical treatment, as well as cases in which at operation no tumour could be found at all. These latter cases could have been examples of endocrine exophthalmos, angiomatous malformations, or orbital cellulitis secondary to an inflammation of an adjacent structure. As we are evaluating only cases in which biopsy material is available, we are concerned only with the third group of the Birch-Hirschfeld classification which comprises patients with exophthalmos in whom a definite tumefaction can be found. These tumours are, however, inflammatory and not neoplastic.Clinically, the condition is characterized by the sudden onset of exophthalmos, by the accompanying lid or conjunctival oedema, by decreased ocular motility, and occasionally by pain or other inflammatory signs. On palpation a hard mass is often found in the orbit.