This study was undertaken to evaluate the ocular hypotensive efficacy of brimonidine Purite (R) 0.15% (Alphagan (R) P 0.15%; Allergan, Inc., Irvine, Calif) given as adjunctive therapy with latanoprost 0.005% (Xalatan (R); Pfizer Inc., New York, NY) to patients with open-angle glaucoma or ocular hypertension. In this multicenter, open-label, prospective evaluation, the intraocular pressure (IOP) of the 43 enrolled patients was >= 18 mm Hg after at least 6 wk of latanoprost mono-therapy. The primary outcome measure was IOP at peak drug effect (10 AM, or approximately 2 h after the morning dose of brimonicline 0.15%). IOP at trough drug effect (8 AM, or approximately 12 h after the evening dose of brimonidine) was also measured. Baseline IOP was 21.9 (+/- 2.3) mm Hg. After 1 mo of treatment, additional mean IOP reductions from latanoprost-treated baseline values were 5.8 mm Hg (26%) at peak drug effect (P <.001) and 3.3 mm Hg (15%) at trough (P <.001). At the month 2 visit, additional mean IOP reductions from latanoprost-treated baseline values were 5.1 mm Hg (23%) at peak drug effect (P <.001) and 2.0 mm Hg (9%) at trough (P=.002). Brimonidine Purite 0.15% provided statistically significant additional reductions in 101? from latanoprost-treated baseline values. These findings suggest that brimonidine Pu rite 0.15% is an efficacious adjunctive therapy in patients given latanoprost who require additional lowering of IOP.