Several reports have documented a contralateral intraocular pressure (IOP) lowering effect after unilateral instillation of timolol. Data derived from the Glaucoma Laser Trial (GLT), a randomized trial of the efficacy and safety of argon laser trabeculoplasty (ALT) as compared with the efficacy and safety of topical medications as initial therapy for primary open-angle glaucoma (POAG), demonstrated an estimated mean pressure reduction of 0.5 mm Hg in the contralateral eye that had received ALT. This contralateral effect is smaller than that observed in other studies of POAG patients. Lower initial IOP, longer duration of treatment with timolol, and/or improved outflow facility modulation in the ALT-treated eyes may be responsible for the smaller contralateral effect of timolol observed in the GLT patients.