We conducted a prospective, randomized, double-masked study to examine the influence of the number of burns administered on intraocular pressure after argon laser trabeculoplasty. Each of 40 patients with open-angle glaucoma had 50 burns placed over one half the trabecular meshwork (Group 1, 20 patients) or 100 burns placed over the entire trabecular meshwork (Group 2, 20 patients). There was no significant difference between the mean intraocular pressure decreases in Group 1 (-9.20 +/- 6.43 mm Hg) and Group 2 (-6.95 +/- 5.74 mm Hg) after two months (P less than .25). In each group, the intraocular pressure was lowest after two months in those patients in whom it was not increased during the immediate postoperative period (Group 1, P less than .006; Group 2, P less than .07). Because the changes in intraocular pressure were statistically similar in the two groups and because patients receiving only 50 burns may have fewer complications, administering 50 laser burns to one half the trabecular meshwork appears to be the better choice for an initial procedure.