We conducted a randomized, double-masked study of 40 patients with open-angle glaucoma to ascertain the immediate intraocular pressure response after treatment of either one half (180 degrees) or the entire (360 degrees) trabecular meshwork by argon laser trabeculoplasty. We found a statistically significant difference in the magnitude of intraocular pressure changes between the two groups (P less than .02), with those patients receiving fewer burns having a smaller increase. Substantial increases in intraocular pressure were observed in five of the 20 patients in whom the entire trabecular meshwork was treated. One of these patients had marked loss of visual field within 24 hours.