Laser trabeculoplasty was performed to treat medically uncontrolled open-angle glaucoma (glaucoma chronicum simplex). Twenty-five eyes were treated with a red krypton laser, while in 110 eyes the usual blue-green argon laser was used, and in 14 eyes a pure green argon laser, under the same conditions. After treatment with the krypton laser the average drop in intraocular pressure at 24 hours was 4.8 mm Hg; at 4 weeks it was 2.3 mm Hg, and at 3 months there was no longer any appreciable effect. In the cases treated by argon laser the pressure drop was significantly greater and lasting, averaging 7.8-8.2 mm Hg from 24 hours after the treatment onward for at least 12 months. There was no difference between the blue-green and the green argon laser as regards pressure drop and improvement in outflow facility.