Purpose To evaluate the nocturnal blood pressure (BP) dipping-pattern in patients with manifest primary open-angle glaucoma (POAG) and to find possible associations with the severity of visual field damage. Methods A number of 314 patients suffering from POAG were consecutively enrolled in this cross-sectional hospital-based study. Each patient had diurnal intraocular pressure (IOP) measurements, 24-hr BP monitoring and computerized perimetry with the Humphrey 30-2 sita Standard program. Inclusion criteria were a mean IOP of less than 15 mmHg with fluctuations of less than 5 mmHg and a visual acuity of at least 20/40. One eye was randomly selected. Based on the night-day BP ratio, a mean arterial nocturnal BP drop of less than 10% was considered as non-dipping, between 10% and 20% as physiological dipping and of more than 20% as over-dipping. Results Glaucoma patients with daytime systemic normotension on the average had more visual field loss in the over-dipper group (MD = - 16.6 dB, IQR = -18.9 to -2.7 dB) than glaucoma patients with daytime systemic hypertension, who had less visual field defects in the over-dipper group (MD = -3.9 dB, IQR = -6.2 to -1.9 dB) (p = 0.004). This result was also found taking age, glaucoma duration, visual acuity, gender, systemic and topical medication as covariates into account. Conclusions To judge the nocturnal BP situation of an individual patient, it is important to do this in relation to the daytime BP level. Twenty-four-hour BP evaluation might be important for all patients with POAG, as nocturnal BP could be a modifiable risk factor for glaucoma severity and progression.