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. The highest IOP during the diurnal curve was 17.4 mmHg (SD 3.00) at 10:00 and the lowest value was 15.0 mmHg (SD 2.7) at 22:00. Seventy-seven per cent of patients had a peak IOP value recorded between 08:00 and 12:00 hours inclusive. The mean peak IOP was 18.3 mmHg (SD 2.6) and the mean trough was 13.1 mmHg (SD 2.2). Thus the mean diurnal range in IOP of 5.2 mmHg (SD 2.2) was similar to that reported by other workers in normals. 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 the pathogenesis of optic nerve damage in all patients with LTG.