Patients referred to a centralized glaucoma laboratory obtained intraocular pressure measurements every two hours from 5:00 am to 3:00 pm. Analysis revealed 400 eyes with visual field defects and 400 eyes without visual field defects as determined by Octopus perimetry of the central 20 degrees. The diurnal variation in intraocular pressure was 6.2 mmHg +/- 3.6 for those with visual field defects and 5.5 mmHg +/- 2.7 for those without visual field defects. There was no statistical significance in the mean diurnal variation between the two groups (P = 0.91). The highest intraocular pressure tended to occur at either 5:00 am to 7:00 am or 11:00 am to 1:00 pm in both groups. The lowest intraocular pressure tended to occur between 7:00 am to 9:00 am or 1:00 pm to 3:00 pm in both groups. No significant differences were noted in the distribution between the two groups with regard to the time of the highest or lowest intraocular pressure. In the group with visual field defects, 30% had an intraocular pressure of less than 23 mmHg and 23% had an intraocular pressure of greater than or equal to 23 mmHg at all five time periods.