To assess the intraocular pressure (IOP) variability in patients with primary open angle glaucoma (POAG) under clinical treatment who reached an established target pressure based on isolated office readings.Retrospective analysis of 65 eyes from 65 POAG patients under clinical therapy who submitted to modified diurnal tension curve (mDTC) (measurements at every 3 hours between 8 am and 5 pm) followed by a water drinking test (WDT). All subjects had established target IOP or =17 mm Hg in 16 of 65 eyes (24.6%). Nine eyes (13.8%) presented values > or =18 mm Hg. The highest IOP detected by mDTC was 20 mm Hg in one patient (1.5%). WDT demonstrated IOP values > or =17 mm Hg in 32 of 65 eyes (49.2%). 22 eyes (33.8%) presented values > or =18 mm Hg after water ingestion. Moreover, IOP levels > or =20 mm Hg were observed in 14 eyes (21.5%).A great percentage of POAG patients undergoing clinical treatment and with IOP control based on single office measurement present significantly higher IOP measurements when performing mDTC and, especially, the WDT.