OBJECTIVES: To determine monthly cost and cost effectiveness of bilateral prostaglandin/ prostamide therapy for lowering intraocular pressure (IOP) in patients taking bimatoprost (0.03%), latanoprost (0.005%), or travoprost (0.004%). METHODS: This prospective pharmacoeconomic study evaluated the direct cost and cost effectiveness of prostaglandin/prostamide therapy for reduction of IOP in patients with glaucoma or ocular hypertension. Drops in five new 2.5-mL bottles were counted and then averaged for each drug. Average retail price was determined by surveys of pharmacies. Drop count, average retail price, average wholesale price, and IOP reduction data were used to compute annual cost, and cost effectiveness (annual cost-per-mm Hg of IOP reduction) of the three drugs.  RESULTS: Drops per 2.5-mL bottle averaged 113 for bimatoprost 0.03% (w/v), 84 for latanoprost 0.005% (w/v), and 83 for travoprost 0.004% (w/v). Average retail cost (2005) per bottle was INR.3,080 for bimatoprost 0.03% (w/v), INR.2,715 for latanoprost 0.005% (w/v), and INR.2,920 for travoprost 0.004% (w/v). The monthly retail cost of bilateral therapy was INR.1,668 for bimatoprost 0.03% (w/v), INR.1,970 for latanoprost 0.005% (w/v), and INR.2,167 for travoprost 0.004% (w/v). Cost effectiveness ranges were INR.2,508 to INR.2,860 per mm Hg reduction in IOP per year for bimatoprost, 0.03% (w/v), INR.2,948 to INR.3,960 per mm Hg for latanoprost 0.005% (w/v), and INR.3,256 to INR.3,696 per mm Hg for travoprost 0.004% (w/v).  CONCLUSIONS: Bimatoprost 0.03% (w/v) had the lowest monthly and annual costs and the greatest cost effectiveness for lowering IOP compared with latanoprost 0.005% (w/v) and travoprost 0.004% (w/v).