The importance of intraocular pressure on the pathogenesis of optic disc damage in normal (low) tension glaucoma has been much debated. This paper looks at the interocular difference in intraocular pressure in an untreated group of 60 patients with normal (low) tension glaucoma. In addition it assesses whether any relationship exists in any one subject between the higher intraocular pressure and the more severe visual field defect. We found that this relationship occurred in 22% (13) of the subjects. In the other 78% (47) of this population no correlation could be found. The existence of such a sizeable subgroup suggests that patients with normal tension glaucoma may be divided into those where intraocular pressure level could be important in the pathogenesis of the visual field defect and a majority where this relationship is less likely.