A retrospective analysis of diurnal variation in intraocular pressure (IOP) in 101 untreated low-tension glaucoma (LTG) patients was carried out to ascertain the role of IOP as a causative factor in the aetiology of optic nerve damage in LTG. The diagnosis of LTG was made only after IOP monitoring as an inpatient, which involved 2 hourly consecutive measurements by Goldmann applanation tonometry from 08:00 to 22:00 hours inclusive. Neither the diurnal pattern nor the range of IOP values seen in this study supports the view that abnormal IOP levels are a significant risk factor in that pathogenesis of optic nerve damage in all patients with LTG.