BACKGROUND: The few investigations that used continuous 24 hour blood pressure monitoring to investigate whether blood pressure in patients with normal tension glaucoma is lower than in normal subjects yielded conflicting results. Therefore, a prospective controlled trial was carried out. METHODS: Systemic blood pressure was recorded continuously over a 24 hour period in 20 patients with normal tension glaucoma (IOP < or = 21 mm Hg). Eight of them showed a localised loss of the neuroretinal rim area and, in addition, optic disc haemorrhages-that is, focal ischaemic signs. Twenty healthy patients without glaucoma, who were hospitalised for cataract or retinal surgery, served as controls. Blood pressure was automatically measured every 20 minutes during the day and every 40 minutes at night. RESULTS: Both groups showed a significant (physiological) blood pressure drop at night, which was significantly (p < 0.001, ANOVA) more pronounced in the group with normal tension glaucoma than in the control group. There was a weak trend towards lower blood pressure values in the normal tension glaucoma group. Minima, maxima, and mean values of the systolic, diastolic, and mean arterial pressures did not differ significantly between the group with normal tension glaucoma and the control group. The greatest differences occurred with nocturnal systolic and diurnal diastolic values. There were no significant differences between the subgroup with focal lesions and the other patients with normal tension glaucoma. CONCLUSIONS: Patients with normal tension glaucoma tend to have lower blood pressure values (p > 0.05, ANOVA) than normals; this difference is probably much smaller than formerly assumed. Patients with normal tension glaucoma, however, have significantly greater nocturnal blood pressure drops (p < 0.001, ANOVA) than normal controls. Nocturnal blood pressure drops (relative day-night differences) may play a more important role in the pathogenesis of normal tension glaucoma than the absolute height of the blood pressure.