The Glaucoma Laser Trial Research Group has examined the acute effects of argon laser trabeculoplasty on immediate change in intraocular pressure and the formation of peripheral anterior synechiae among the 271 eyes assigned to argon laser trabeculoplasty as initial treatment for primary open angle glaucoma. Argon laser trabeculoplasty was administered in two sessions. Intraocular pressure rises of greater than 5 mm Hg occurred in 34% of eyes after one or both of the treatment sessions; intraocular pressure rises of greater than 10 mm Hg occurred in 12%. Eyes that had an intraocular pressure rise after the first session were more likely to have a rise after the second session than eyes with no rise after the first session. Among the many patient, eye, and treatment characteristics examined, only pigmentation of the trabecular meshwork was strongly associated with intraocular pressure rises. Forty-six percent of eyes developed greater than or equal to 1 degree of peripheral anterior synechiae. Brown iris color and posterior placement of laser burns were associated with higher rates of peripheral anterior synechiae formation.