Editor,—The aetiology of normal tension glaucoma (NTG) is still debatable. Abnormal blood flow, systemic hypotension, abnormal blood coagulability, and other factors associated with cerebrovascular disease may have a causative role in NTG.A study was designed to look at the prevalence of familial thrombophilia in cases of NTG. ### CASE REPORT Seventy two patients were identified from ophthalmological database records with the diagnosis of NTG (defined as intraocular pressure <21 mm Hg, open drainage angle on gonioscopy, absence of any secondary cause for a glaucomatous optic neuropathy, and typical optic disc cupping which correlates with the visual field loss). Strict criteria were used for entry into the study. Patients with NTG had to be under the age of 70 years with normal computerised tomography and normal day time intraocular pressure phasing. Forty five patients did not fulfil these criteria and so were excluded. Twenty seven patients formed the study group. None of these patients were on any medication which would be expected to have altered the values …