Purpose To assess the diurnal and nocturnal persistency of intraocular pressure (IOP) reduction after omission of up to two doses of once-daily topical travoprost in patients with open-angle glaucoma or ocular hypertension. Design Prospective, open-label study. Methods Twenty subjects underwent three sessions of 24-hour IOP monitoring. The first session occurred before initiating treatment of newly diagnosed patients or after a four-week washout in patients already receiving medical therapy. The second session occurred after four weeks or more of travoprost treatment. The third session was performed 41 to 63 hours after the last travoprost dose. Results IOP lowering persisted after omission of one to two doses. Between 41 to 63 hours after the last dose, diurnal IOP reduction was attenuated, but nocturnal IOP reduction sustained. Conclusions IOP lowering effect after omission of one to two doses of travoprost is attenuated in the diurnal period but sustained in the nocturnal period, the time corresponding to the highest baseline habitual IOP.