PURPOSE: Intraocular pressure has been shown in past studies to vary diurnally. The purpose of this study was to establish whether patients treated with timolol maleate for primary open-angle glaucoma should be reviewed at a specific time of day, within normal clinical hours (9 a.m. to 5 p.m.). It is normal practice to schedule review appointments into either morning or afternoon clinic sessions. This may result in abnormally high intraocular pressure values being missed. METHODS: The intraocular pressure of 28 people with primary open-angle glaucoma was measured by means of a Goldmann applanator on an hourly basis between the hours of 9 a.m. and 5 p.m. All subjects continued their antiglaucoma therapy of timolol maleate throughout the measurement period. RESULTS: The intraocular pressure increased significantly (p = 0.0008) throughout the measurement period, with maximal intraocular pressure values occurring at 5 p.m. in the afternoon. CONCLUSION: Although monitoring intraocular pressure values should not be the only consideration in glaucoma management, undetected rises in pressure may precede the development of optic disc cupping or visual field loss. The variability of intraocular pressure values has important implications in the scheduling of glaucoma intraocular pressure review measurements for the purposes of continuing treatment.