The Glaucoma Laser Trial, a multicenter, randomized clinical trial involving 271 patients, was designed to assess the efficacy and safety of argon laser trabeculoplasty (ALT) as an alternative treatment with topical medication for controlling intraocular pressure (IOP) in patients with newly diagnosed, previously, untreated primary open-angle glaucoma (POAG). Each patient had one eye randomly assigned to ALT (the laser first [LF] eye) and the other eye assigned to timolol maleate 0.5% (the medication first [MF] eye). Medication was initiated or changed for either eye according to the same stepped regimen if the IOP was not controlled. Throughout the 2-year follow-up, LF eyes had lower mean IOPs than MF eyes (1-2 mmHg), and fewer LF eyes than MF eyes required simultaneous prescription of two or more medications to control IOP (P less than 0.001). After 2 years of follow-up, 44% of LF eyes were controlled by ALT, 70% were controlled by ALT or ALT and timolol, and 89% were controlled within the stepped medication regimen. After 2 years, 30% of MF eyes remained controlled by timolol, and 66% were controlled within the stepped regimen. There were no major differences between the two treatment approaches with respect to changes in visual acuity or visual field over the 2 years of follow-up.